Use Refineable for ThemeStyles (#3196)

This PR updates the `ThemeStyles` struct to use the `Refineable` trait
instead of a custom declarative macro for generating refinements.

Release Notes:

- N/A

crates/theme2/src/syntax.rs

@@ -1,6 +1,6 @@
 use gpui2::{HighlightStyle, Hsla};
 
-#[derive(Clone)]
+#[derive(Clone, Default)]
 pub struct SyntaxTheme {
     pub highlights: Vec<(String, HighlightStyle)>,
 }