my lightdm gtk greeter.css

/*
my indicator in lightdm-gtk-greeter.conf

indicators = ~host;~power;~spacer;~~Arch Linux;~spacer;~session;~clock
*/

@define-color bg rgba(20, 45, 50, 0.9);
@define-color bg_darken rgba(20, 45, 50, 0.75);
@define-color clear rgba(0, 0, 0, 0);
@define-color bg_alt #b13041;
@define-color fg #c0e0f0;

* {
	background-clip: padding-box;
	-GtkToolButton-icon-spacing: 4;
	-GtkDialog-button-spacing: 4;
	-GtkDialog-action-area-border: 0;
	-gtk-outline-radius: 0px;
	outline-style: none;
	outline-offset: 0px;
	outline-width: 0px;
	color: @fg;
	font-weight: bold;
}

#panel_window {
  background-color: @clear;
}

#panel_window menubar {
	border: solid @clear;
	border-width: 20px 50px 20px 50px;
}

#panel_window menubar > menuitem, 
#panel_window menubar > separator {
	border: 11px solid @bg;
	background-color:  @bg;
	color: inherit; 
}

#panel_window menubar > menuitem {
	background-color: @bg_alt;
	border-color: @bg_alt;
	color: @bg;
}

#panel_window menubar menuitem:disabled {
	color: inherit; 
}

#panel_window menubar menuitem:disabled label {
	color: inherit; 
}

#panel_window menubar menu > menuitem {
	border: 8px solid @bg_darken;
	background-color: @bg_darken;
}

#content_frame {
	padding-bottom: 15px;
	background-color: @bg;
	border: solid @bg_darken;
	border-width: 8px 8px 0px 8px;
}

#content_frame button, 
#content_frame button:hover,
#content_frame button:active, 
#content_frame button:checked,
#content_frame button:disabled {
	background-color: @clear;
}

#buttonbox_frame {
	padding-top: 5px;
	padding-bottom: 0px;
	background-color: @bg;
	border: solid @bg_darken;
	border-width: 0px 8px 8px 8px;
}

#buttonbox_frame button, 
#buttonbox_frame button:hover,
#buttonbox_frame button:active, 
#buttonbox_frame button:checked,
#buttonbox_frame button:disabled {
	background-color: @bg_alt;
	border: 1px solid @bg_darken;
	padding: 8px 15px;
	min-width: 70px;
}

#login_window #user_combobox,
#login_window #username_entry,
#login_window #password_entry {
	padding: 8px 0px 8px 0px;
}

#login_window #user_combobox menu {
	padding: 8px;
	background-color: @bg_darken;
}

Hi, thanks for sharing. But where should I edit CSS?

Same, where is the css located ?

It seems the only way to use a CSS for lightdm-gtk-greeter is to append, within a relevant css file of your selected theme-- ie /usr/share/themes/themename/gtk-3.0/gtk.css (neither /etc/gtk-3.0/gtk.css nor /root/.config/gtk-3.0/gtk.css appear to be acknowledged by lightdm-gtk-greeter)--an @import to a file named "lightdm-gtk-greeter.css" (no logical reason why it has to be named that; but just in case and for clarity), such as:

@import url("lightdm-gtk-greeter.css");

For which case I've placed lightdm-gtk-greeter.css within the theme's directory.

What's unfortunate about this is that it means the next time your theme package updates, your lightdm theme will be disabled (the css file will be overwritten, removing this @import line.