Opinionated theme for Obsidian. Tested only on macOS.

obsidian-theme.css

body {
	--status-bar-background: transparent;
	--status-bar-border-width: 0;
	--status-bar-text-color: var(--text-faint);
	--tab-radius: var(--radius-m);
	--tab-radius-active: var(--radius-m);
	--checkbox-size: 1.3em;
	--ribbon-background: transparent;
	--tab-outline-color: transparent;
	--titlebar-background: transparent;
	--titlebar-background-focused: transparent;
	--ribbon-background: transparent;
	--ribbon-background-collapsed: transparent;
	--background-secondary: transparent;
	--tab-container-background: transparent;
	--checklist-done-color: var(--text-faint);
	--divider-color: transparent !important;
}

.is-fullscreen {
	background-color: var(--background-primary) !important;
}

.theme-light,
.theme-dark.is-fullscreen {
	--workspace-background-translucent: var(--background-primary);
}

.theme-dark:not(.is-fullscreen) {
	--workspace-background-translucent: rgba(30, 30, 30, 0.8);
}

/* Fullscreen Layout Tweaks */
.is-fullscreen .workspace-ribbon.mod-left {
	margin-top: unset;
}

/* Hide sidebar toggle buttons */
.sidebar-toggle-button {
	display: none;
}

/* Unset backgrounds of main areas */
.workspace-split.mod-root,
.workspace-split.mod-root .view-content {
	background-color: transparent !important;
}

/* Hide Tab UI */
.workspace-tab-header-new-tab,
.workspace-tab-header-tab-list {
	display: none !important;
}

.workspace-split.mod-root .workspace-tab-header.is-active::before,
.workspace-split.mod-root .workspace-tab-header.is-active::after {
	display: none;
}

.workspace-tab-header {
	opacity: 0.2;
}

.workspace-tab-header .workspace-tab-header-inner-close-button {
	opacity: 0 !important;
}

.workspace-tab-header:hover .workspace-tab-header-inner-close-button {
	opacity: 1 !important;
}

/* Remove top space from side panels */
.is-fullscreen
	.is-left-sidedock-open
	.mod-top-left-space
	.workspace-tab-header-container {
	display: none !important;
}

.is-right-sidedock-open .mod-top-right-space .workspace-tab-header-container {
	display: none !important;
}

/* Make icons less visible */
.side-dock-settings,
.side-dock-actions {
	opacity: 0.4;
}

/* Typesetting for Headers */
.inline-title,
.HyperMD-header {
	letter-spacing: -0.02em;
	line-height: 1.4;
	font-weight: 700;
}

.HyperMD-header {
	padding-top: 0.6em !important;
	padding-bottom: 0.4em !important;
}

/* Rounded Corners for Media */
.workspace-leaf-content img:not([width]),
.workspace-leaf-content video {
	border-radius: 8px;
}