theme: Use a matching layout theme for any widget theme

Dedicated styling is now possible for themes which have a corresponding style-theme.css file. Adwaita:dark gets one, whereas other themes use the new generic fallback theme.

src/lib.rs

@@ -24,6 +24,7 @@ mod outputs;
 mod popover;
 mod resources;
 mod submission;
+mod style;
 pub mod tests;
 pub mod util;
 mod xdg;