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/button.rs

@@ -41,8 +41,8 @@ pub enum ButtonVariant {
 impl ButtonVariant {
     pub fn bg_color(&self, cx: &mut WindowContext) -> Hsla {
         match self {
-            ButtonVariant::Ghost => cx.theme().colors().ghost_element,
-            ButtonVariant::Filled => cx.theme().colors().element,
+            ButtonVariant::Ghost => cx.theme().colors().ghost_element_background,
+            ButtonVariant::Filled => cx.theme().colors().element_background,
         }
     }