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/panes.rs

@@ -113,7 +113,7 @@ impl<V: 'static> PaneGroup<V> {
                 .gap_px()
                 .w_full()
                 .h_full()
-                .bg(cx.theme().colors().editor)
+                .bg(cx.theme().colors().editor_background)
                 .children(self.groups.into_iter().map(|group| group.render(view, cx)));
 
             if self.split_direction == SplitDirection::Horizontal {