theme: Remove default syntax colors (#11980)

This PR removes the default syntax colors from the theme.

With the changes in #11911 these colors could leak through if the theme
didn't provide a value for that syntax color.

Removing them gives themes a clean slate to work with.

Release Notes:

- N/A

crates/theme/src/default_theme.rs

@@ -22,7 +22,7 @@ fn zed_pro_daylight() -> Theme {
             colors: ThemeColors::light(),
             status: StatusColors::light(),
             player: PlayerColors::light(),
-            syntax: Arc::new(SyntaxTheme::light()),
+            syntax: Arc::new(SyntaxTheme::default()),
             accents: vec![
                 blue().light().step_9(),
                 orange().light().step_9(),
@@ -53,7 +53,7 @@ pub(crate) fn zed_pro_moonlight() -> Theme {
             colors: ThemeColors::dark(),
             status: StatusColors::dark(),
             player: PlayerColors::dark(),
-            syntax: Arc::new(SyntaxTheme::dark()),
+            syntax: Arc::new(SyntaxTheme::default()),
             accents: vec![
                 blue().dark().step_9(),
                 orange().dark().step_9(),