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/workspace2/src/workspace2.rs

@@ -2697,7 +2697,7 @@ impl Workspace {
 
     fn render_titlebar(&self, cx: &mut ViewContext<Self>) -> impl Component<Self> {
         div()
-            .bg(cx.theme().colors().title_bar)
+            .bg(cx.theme().colors().title_bar_background)
             .when(
                 !matches!(cx.window_bounds(), WindowBounds::Fullscreen),
                 |s| s.pl_20(),