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/components/panel.rs

@@ -107,7 +107,7 @@ impl<V: 'static> Panel<V> {
                 PanelSide::Right => this.border_l(),
                 PanelSide::Bottom => this.border_b().w_full().h(current_size),
             })
-            .bg(cx.theme().colors().surface)
+            .bg(cx.theme().colors().surface_background)
             .border_color(cx.theme().colors().border)
             .children(self.children)
     }