In GTK+ 3.0, GtkStyleContext was added to replace GtkStyle and the theming infrastructure available in 2.x. GtkStyleContext is an object similar in spirit to GtkStyle, as it contains theming information, although in a more complete and tokenized fashion. There are two aspects to switching to GtkStyleContext: porting themes and theme engines, and porting applications, libraries and widgets.
From GTK+ 3.0 on, theme engines must implement GtkThemingEngine and be
and the files containing style information must be written in the CSS-like
format that is understood by GtkCssProvider. For a theme named
"Clearlooks", the CSS file parsed by default is
with possible variants such as the dark theme being named
gtk-dark.css in the same directory.
If your theme RC file was providing values for GtkSettings, you
can install a
settings.ini keyfile along with
gtk.css to provide theme-specific defaults
Key themes have been converted to CSS syntax too. See the
information about the syntax. GTK+ looks for key themes in the file
theme is the current
key theme name.