Theming changes: GTK+ 3 Reference Manual

## Theming changes

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.

## Migrating themes

From GTK+ 3.0 on, theme engines must implement GtkThemingEngine and be installed in `$libdir/gtk+-3.0/$GTK_VERSION/theming-engines`, 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 `$datadir/themes/Clearlooks/gtk-3.0/gtk.css`, 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 the `gtk.css` to provide theme-specific defaults for settings.

Key themes have been converted to CSS syntax too. See the GtkCssProvider documentation information about the syntax. GTK+ looks for key themes in the file `$datadir/themes/`theme`/gtk-3.0/gtk-keys.css`, where `theme` is the current key theme name.