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/icon_button.rs

@@ -73,12 +73,12 @@ impl<V: 'static> IconButton<V> {
 
         let (bg_color, bg_hover_color, bg_active_color) = match self.variant {
             ButtonVariant::Filled => (
-                cx.theme().colors().element,
+                cx.theme().colors().element_background,
                 cx.theme().colors().element_hover,
                 cx.theme().colors().element_active,
             ),
             ButtonVariant::Ghost => (
-                cx.theme().colors().ghost_element,
+                cx.theme().colors().ghost_element_background,
                 cx.theme().colors().ghost_element_hover,
                 cx.theme().colors().ghost_element_active,
             ),