elementaryOS 6 has great dark themes, but they only apply to "curated" appcenter apps. This script forces the system theme to work with regular Debian packages as well as non-curated Flatpak apps. It also optionally changes the wallpaper based on the theme. It also supports changing themes for the Mailspring email app (both .deb and Snap versions).

eOSUDS.sh

#!/usr/bin/bash


# =======================================
# eOS-Universal-Dark-Style
# =======================================

# elementaryOS 6 has great dark themes, but they only apply to "curated" appcenter apps
# This script forces the system theme to work with regular Debian packages as well as non-curated Flatpak apps
# It also optionally changes the wallpaper based on the theme
# It also supports changing themes for the Mailspring email app (both .deb and Snap versions)


# =======================================
# Instructions
# =======================================

# 1. Save this script and make it executable
# 2. Change the configuration variables below as needed
# 3. Use a cron job to run the script automatically on a desired schedule; you can learn about cron by running `man crontab` in a terminal
#    Note that system updates can reset the forced Flatpak theming, so running the script frequently (every few minutes) is recommended

# =======================================
# Configuration
# =======================================

LIGHT_THEME_WALLPAPER='/usr/share/backgrounds/odin.jpg' # Leave empty to disable wallpaper switching on light theme
DARK_THEME_WALLPAPER='/usr/share/backgrounds/odin-dark.jpg' # Leave empty to disable wallpaper switching on dark theme
MAILSPRING_LIGHT_THEME='ui-light' # Leave empty to disable Mailspring theme switching on light theme
MAILSPRING_DARK_THEME='ui-dark' # Leave empty to disable Mailspring theme switching on dark theme
SHOW_DEBUG_MESSAGES=0 # If debug logs should be printed, set to 1


# =======================================
# Script functions
# =======================================

# Echoes provided string with a date prefix. If $2 is 1, redirects to stderr with ERROR prefix. If $3 is 1, echoes a crash message to stderr and exits with code 1
# If $4 is 1, the message is treated as a debug log (only shown if the relevant configuration variable above is set)
log() {
	if [ "$2" == 1 ]; then
		echo -e "ERROR: $(date): $1" >&2
	elif [ "$4" == 1 ]; then
		if [ "$SHOW_DEBUG_MESSAGES" == 1 ]; then
			echo -e "DEBUG: $(date): $1"
		fi
	else
		echo -e "MESSG: $(date): $1"
	fi
	if [ "$3" == 1 ]; then
		echo -e "CRASHED." >&2
		exit 1
	fi

}

# Echoes 1 if the system dark style is set, 0 otherwise
getSystemDarkStylePref() {
	[ "$(gsettings get org.freedesktop prefers-color-scheme)" = "'dark'" ] && echo 1 || echo 0
}

# Changes the universal GTK dark theme preference based on the provided argument (if they are different)
changeGTKUniversalDarkPrefIfNeeded() {
	# Ensure the provided argument is valid before making the change
	[ "$1" == 0 ] || [ "$1" == 1 ] || log "GTK universal dark preference can only be set to 0 or 1" 1 1
	# The location of the universal GTK dark preference config file
	GTK_UNIVERSAL_PREF_FILE=~/.config/gtk-3.0/settings.ini
	# Ensure the file exists (if not, create it)
	if [ ! -f "$GTK_UNIVERSAL_PREF_FILE" ]; then
		echo -e '[Settings]\ngtk-application-prefer-dark-theme=1\n' > "$GTK_UNIVERSAL_PREF_FILE"
	fi
	# Grab the pref
	GTK_UNIVERSAL_PREF_IS_DARK="$(cat "$GTK_UNIVERSAL_PREF_FILE" | grep -o [0-1])"
	# Ensure it is 0 or 1; if not, log the error and exit
	[ "$GTK_UNIVERSAL_PREF_IS_DARK" == 0 ] || [ "$GTK_UNIVERSAL_PREF_IS_DARK" == 1 ] || log "$GTK_UNIVERSAL_PREF_FILE is not valid" 1 1
	# If needed, make the change
	if [ "$GTK_UNIVERSAL_PREF_IS_DARK" != "$1" ]; then
		sed -i "s/gtk-application-prefer-dark-theme=.*/gtk-application-prefer-dark-theme=$1/" "$GTK_UNIVERSAL_PREF_FILE"
		log "GTK universal dark preference has been set to $1"
	else
		log "GTK universal dark preference is already $1, no changes made" 0 0 1
	fi
}

# Changes the wallpaper to the one given in the argument (if not already set)
# If the argument is empty, does nothing (does NOT throw an error)
changeWallpaperIfNeeded() {
	if [ ! -z "$1" ]; then
		CURRENT_WALLPAPER="$(gsettings get org.gnome.desktop.background picture-uri | head -c -2 | tail -c +9)"
		if [ "$CURRENT_WALLPAPER" != "$1" ]; then
			# This if statement allows the wallpaper change to work even when running from a cron job
			if [ -z "$DBUS_SESSION_BUS_ADDRESS" ]; then
				PID=$(pgrep gnome-session)
				export DBUS_SESSION_BUS_ADDRESS=$(grep -z DBUS_SESSION_BUS_ADDRESS /proc/$PID/environ | tr '\0' '\n' | cut -d= -f2-)
			fi
			gsettings set org.gnome.desktop.background picture-uri file://"$1"
			log "Wallpaper has been set to $1"
		else
			log "Wallpaper is already $1, no changes made" 0 0 1
		fi
	else
		log "Wallpaper argument is empty, no changes made" 0 0 1
	fi
}

# Changes the Mailspring theme to the provided string if a Mailspring config can be found (indicating it is installed)
# If not installed or if no theme provided, does nothing (does NOT throw an error)
# Warning: This abruptly kills the Mailspring process and restarts it in the background
changeMailSpringThemeIfNeeded() {
	if [ ! -z "$1" ]; then
		MAILSPRING_CONFIG_FILE=~/.config/Mailspring/config.json
		if [ ! -f "$MAILSPRING_CONFIG_FILE" ]; then	MAILSPRING_CONFIG_FILE=~/snap/mailspring/common/config.json; fi
		if [ -f "$MAILSPRING_CONFIG_FILE" ] && [ ! -z "$1" ]; then
			CURRENT_MAILSPRING_THEME="$(cat "$MAILSPRING_CONFIG_FILE" | grep \"theme\": | tr -d ' ' | tail -c +10 | head -c -3)"
			if [ "$CURRENT_MAILSPRING_THEME" != "$1" ]; then
				sed -i "s/\"theme\":.*/\"theme\": \"$1\",/g" "$MAILSPRING_CONFIG_FILE"
				log "Mailspring theme has been set to $1"
				pkill -f mailspring > /dev/null 2>&1
				log "Mailspring process has been killed"
				mailspring -b </dev/null &>/dev/null &
				log "Mailspring process has been started in background"
			else
				log "Mailspring theme is already $1, no changes made" 0 0 1
			fi
		else
			log "Mailspring not installed, no changes made" 0 0 1
		fi
	else
		log "Mailspring theme argument is empty, no changes made" 0 0 1
	fi
}

# First, installs the current system theme into the Flatpak runtimes used by all non-curated (non-appcenter) apps (if not already installed)
# If the current theme is an elementary theme (io.elementary.stylesheet.*) and if the $1 argument is 1, it makes these changes before installing the theme:
# - Renames it by adding a -dark suffix
# - Modifies it so the default stylesheet is dark themed; this is needed as elementary themes do not have traditional separate dark variants
# Then, forces all the apps to use the current theme by adding an environment variable override (if not already set)
changeNonElementaryFlatpakThemesIfNeeded() {
	# The location of Flatpak runtimes
	FLATPAK_RUNTIME_DIR=~/.local/share/flatpak/runtime
	# Where themes are stored in a Flatpak runtime
	RUNTIME_THEME_SUBDIR='active/files/share/themes'
	# Grab the current system theme
	THEME="$(gsettings get org.gnome.desktop.interface gtk-theme | head -c -2 | tail -c +2)"
	# Set the variable that indicates whether to add the '-dark' prefix to the theme based on the $1 argument and whether the current theme is an elementary one
	USE_CUSTOM_ELEMENTARY_DARK="$([ "$1" == 1 ] && [ ! -z "$(echo "$THEME" | grep -e "^io\.elementary\.stylesheet\..*")" ] && echo 1 || echo 0)"
	# Grab the list of app IDs for all non-appcenter installed Flatpaks
	TARGET_APP_IDS=($(flatpak list --columns=application,origin,runtime | awk -F "\t" '{if ($2 != "appcenter"  && $3 != "") print $1}'))
	# For each target app ID...
	for APP_ID in "${TARGET_APP_IDS[@]}"; do
		# Grab the appropriate Flatpak runtime
		RUNTIME="$(flatpak info --show-runtime "$APP_ID")"
		# Ensure the runtime has a theme directory
		RUNTIME_THEME_DIR="$FLATPAK_RUNTIME_DIR"/"$RUNTIME"/"$RUNTIME_THEME_SUBDIR"
		if [ ! -d "$RUNTIME_THEME_DIR" ]; then mkdir -p "$RUNTIME_THEME_DIR"; fi
		# Check to see if the current theme (or the current theme plus the '-dark' suffix if needed) is already installed in the runtime
		THEME_IN_RUNTIME_SEARCH_RESULT="$([ "$USE_CUSTOM_ELEMENTARY_DARK" -eq 1 ] && echo "$(ls "$RUNTIME_THEME_DIR" | grep "$THEME"-dark)" || echo "$(ls "$RUNTIME_THEME_DIR" | grep "$THEME")")"
		# If not, find where the theme is stored (in the system or user directory) and copy it into the runtime theme folder
		# Add '-dark' suffix to it if needed and make the necessary changes
		if [ -z "$THEME_IN_RUNTIME_SEARCH_RESULT" ]; then
			# Find where the theme is stored
			TARGET_THEME_LOCATION=''
			POSSIBLE_THEME_LOCATIONS=(/usr/share/themes ~/.local/share/themes)
			for LOCATION in "${POSSIBLE_THEME_LOCATIONS[@]}"; do
				if [ ! -z "$(ls "$LOCATION" | grep -e ^"$THEME"$)" ]; then TARGET_THEME_LOCATION="$LOCATION"/"$THEME"; fi
			done
			[ ! -z "$TARGET_THEME_LOCATION" ] || log "Location of the currently set theme - $THEME - could not be found" 1 1
			# If there is not need to make the '-dark' modifications, copy the theme into the Flatpak runtime; else, modify it in a temporary directory first, then copy it
			if [ ! "$USE_CUSTOM_ELEMENTARY_DARK" -eq 1 ]; then
				cp -r "$TARGET_THEME_LOCATION" "$RUNTIME_THEME_DIR"
				log "$THEME theme has been installed into $RUNTIME Flatpak runtime"
			else
				TEMP_DIR="$(mktemp -d)"
				cp -r "$TARGET_THEME_LOCATION" "$TEMP_DIR"
				cp -r "$TEMP_DIR"/"$THEME" "$TEMP_DIR"/"$THEME"-dark
				mv "$TEMP_DIR"/"$THEME"-dark/plank "$TEMP_DIR"/"$THEME"-dark/plank-light
				mv "$TEMP_DIR"/"$THEME"-dark/plank-dark "$TEMP_DIR"/"$THEME"-dark/plank
				mv "$TEMP_DIR"/"$THEME"-dark/gtk-3.0/gtk.css "$TEMP_DIR"/"$THEME"-dark/gtk-3.0/gtk-light.css
				mv "$TEMP_DIR"/"$THEME"-dark/gtk-3.0/gtk-dark.css "$TEMP_DIR"/"$THEME"-dark/gtk-3.0/gtk.css
				cp -r "$TEMP_DIR"/"$THEME"-dark "$RUNTIME_THEME_DIR"
				rm -r "$TEMP_DIR"
				log "$THEME theme has been modified to $THEME-dark and installed into $RUNTIME Flatpak runtime"
			fi
		else
			log "$THEME theme is already installed in the $RUNTIME Flatpak runtime, no changes made" 0 0 1
		fi
		# Now that the theme is installed, do the override if needed
		CURRENT_APP_THEME="$(flatpak override --user --show "$APP_ID" | grep GTK_THEME= | tail -c +11)"
		FINAL_OVERRIDE_THEME_NAME="$([ "$USE_CUSTOM_ELEMENTARY_DARK" == 1 ] && echo "$(echo "$THEME" | head -c -1 | tail -c +1)-dark" || echo "$THEME")"
		if [ "$CURRENT_APP_THEME" != "$FINAL_OVERRIDE_THEME_NAME" ]; then
			flatpak override --user --env=GTK_THEME="$FINAL_OVERRIDE_THEME_NAME" "$APP_ID"
			log "Flatpak environment variable override 'GTK_THEME=$FINAL_OVERRIDE_THEME_NAME' has been added for $APP_ID"
		else
			log "Flatpak environment variable override 'GTK_THEME=$FINAL_OVERRIDE_THEME_NAME' is already set for $APP_ID, no changes made" 0 0 1
		fi
	done
}


# =======================================
# Actual execution
# =======================================

# 1. Change the universal GTK dark theme preference if needed
# 2. Change the wallpaper if needed
# 3. Change the Mailspring theme if needed
# 4. Change the theme of all non-elementary Flatpak apps if needed 

SYSTEM_DARK_PREF="$(getSystemDarkStylePref)"
changeGTKUniversalDarkPrefIfNeeded "$SYSTEM_DARK_PREF"
changeWallpaperIfNeeded "$([ "$SYSTEM_DARK_PREF" == 1 ] && echo "$DARK_THEME_WALLPAPER" || echo "$LIGHT_THEME_WALLPAPER")"
changeMailSpringThemeIfNeeded "$([ "$SYSTEM_DARK_PREF" == 1 ] && echo "$MAILSPRING_DARK_THEME" || echo "$MAILSPRING_LIGHT_THEME")"
changeNonElementaryFlatpakThemesIfNeeded "$SYSTEM_DARK_PREF"

I am not able to apply it

This is what i am getting.
can u tell me how to fix this.
I very much appreciate u work. Have a good day.

can u tell me how to fix this.

Should be fixed now.

im getting the same error too

im getting the same error too

Should be fixed now. It should now create the settings.ini file if needed.

It looks like it stopped detecting theme changes. :(

Also, sometimes it will show me this error:

ERROR: vie 27 ago 2021 20:14:07 CDT: /home/mrtn/.config/gtk-3.0/settings.ini is not valid
CRASHED.

What about making this script more cron-friendly? Better than doing the checks in an infinite while loop.

Edit: About the error, it looks like sometimes it will randomly append a 0 to the dark preference theme, so that's when it gets triggered.

What about making this script more cron-friendly? Better than doing the checks in an infinite while loop.

Edit: About the error, it looks like sometimes it will randomly append a 0 to the dark preference theme, so that's when it gets triggered.

Yep, turns out extra newlines in the settings.ini file were causing problems. Fixed now, thanks for pointing that out.
Also you're right about the while loop; no point making a custom timing function when Linux already has a widely used solution. Have made the changes.

still not working, test on eos 6.

Moving forward, elementaryOS's dark theme preference will be natively supported by GTK apps. Not sure what the status of that is since I'm not using eOS anymore, but you could look into that. Was working fine for me on eOS 6 a few months ago, so I'm not sure what changed. Probably won't make any more changes to this script.

Moving forward, elementaryOS's dark theme preference will be natively supported by GTK apps. Not sure what the status of that is since I'm not using eOS anymore, but you could look into that. Was working fine for me on eOS 6 a few months ago, so I'm not sure what changed. Probably won't make any more changes to this script.

thanks for your repply. Eos6 still not support dark mode in flatpak apps.