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/keybinding.rs

@@ -66,7 +66,7 @@ impl Key {
             .rounded_md()
             .text_sm()
             .text_color(cx.theme().colors().text)
-            .bg(cx.theme().colors().element)
+            .bg(cx.theme().colors().element_background)
             .child(self.key.clone())
     }
 }