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/indicator.rs

@@ -14,7 +14,7 @@ impl UnreadIndicator {
         div()
             .rounded_full()
             .border_2()
-            .border_color(cx.theme().colors().surface)
+            .border_color(cx.theme().colors().surface_background)
             .w(px(9.0))
             .h(px(9.0))
             .z_index(2)