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/checkbox.rs

@@ -120,7 +120,10 @@ impl Checkbox {
                 cx.theme().colors().element_selected,
                 cx.theme().colors().border,
             ),
-            (false, false) => (cx.theme().colors().element, cx.theme().colors().border),
+            (false, false) => (
+                cx.theme().colors().element_background,
+                cx.theme().colors().border,
+            ),
         };
 
         div()