Rename fields in ThemeColors (#3242)

This PR applies a number of field renames in the `ThemeColors` struct
from the `import-theme` branch.

This will help prevent this branch from diverging too far from `main`.

Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Marshall Bowers <1486634+maxdeviant@users.noreply.github.com>

crates/ui2/src/to_extract/toolbar.rs

@@ -56,7 +56,7 @@ impl<V: 'static> Toolbar<V> {
 
     fn render(self, _view: &mut V, cx: &mut ViewContext<V>) -> impl Component<V> {
         div()
-            .bg(cx.theme().colors().toolbar)
+            .bg(cx.theme().colors().toolbar_background)
             .p_2()
             .flex()
             .justify_between()