GtkCssProvider is an object implementing the #GtkStyleProvider interface.
It is able to parse [CSS-like][css-overview] input in order to style widgets.
In addition, certain files will be read when GTK+ is initialized. First, the
file $XDG_CONFIG_HOME/gtk-3.0/gtk.css is loaded if it exists. Then, GTK+
loads the first existing file among
XDG_DATA_HOME/themes/THEME/gtk-VERSION/gtk.css,
$HOME/.themes/THEME/gtk-VERSION/gtk.css,
$XDG_DATA_DIRS/themes/THEME/gtk-VERSION/gtk.css and
DATADIR/share/themes/THEME/gtk-VERSION/gtk.css, where THEME is the name of
the current theme (see the #GtkSettings:gtk-theme-name setting), DATADIR
is the prefix configured when GTK+ was compiled (unless overridden by the
GTK_DATA_PREFIX environment variable), and VERSION is the GTK+ version number.
If no file is found for the current version, GTK+ tries older versions all the
way back to 3.0.
In the same way, GTK+ tries to load a gtk-keys.css file for the current
key theme, as defined by #GtkSettings:gtk-key-theme-name.
GtkCssProvider is an object implementing the #GtkStyleProvider interface. It is able to parse [CSS-like][css-overview] input in order to style widgets.
An application can make GTK+ parse a specific CSS style sheet by calling gtk.css_provider.CssProvider.loadFromFile or gtk.css_provider.CssProvider.loadFromResource and adding the provider with gtk.style_context.StyleContext.addProvider or gtk.style_context.StyleContext.addProviderForScreen.
In addition, certain files will be read when GTK+ is initialized. First, the file $XDG_CONFIG_HOME/gtk-3.0/gtk.css is loaded if it exists. Then, GTK+ loads the first existing file among XDG_DATA_HOME/themes/THEME/gtk-VERSION/gtk.css, $HOME/.themes/THEME/gtk-VERSION/gtk.css, $XDG_DATA_DIRS/themes/THEME/gtk-VERSION/gtk.css and DATADIR/share/themes/THEME/gtk-VERSION/gtk.css, where THEME is the name of the current theme (see the #GtkSettings:gtk-theme-name setting), DATADIR is the prefix configured when GTK+ was compiled (unless overridden by the GTK_DATA_PREFIX environment variable), and VERSION is the GTK+ version number. If no file is found for the current version, GTK+ tries older versions all the way back to 3.0.
In the same way, GTK+ tries to load a gtk-keys.css file for the current key theme, as defined by #GtkSettings:gtk-key-theme-name.