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/project_panel.rs

@@ -19,7 +19,7 @@ impl ProjectPanel {
             .flex()
             .flex_col()
             .size_full()
-            .bg(cx.theme().colors().surface)
+            .bg(cx.theme().colors().surface_background)
             .child(
                 div()
                     .id("project-panel-contents")