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/tab.rs

@@ -109,12 +109,12 @@ impl Tab {
 
         let (tab_bg, tab_hover_bg, tab_active_bg) = match self.current {
             false => (
-                cx.theme().colors().tab_inactive,
+                cx.theme().colors().tab_inactive_background,
                 cx.theme().colors().ghost_element_hover,
                 cx.theme().colors().ghost_element_active,
             ),
             true => (
-                cx.theme().colors().tab_active,
+                cx.theme().colors().tab_active_background,
                 cx.theme().colors().element_hover,
                 cx.theme().colors().element_active,
             ),