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.