ui2: Reference theme2 and settings2 crates without the 2 (#3424)

This PR updates the `ui2` crate to reference the `theme2` and
`settings2` crates without the `2` the way we do in our other crates.

Release Notes:

- N/A

crates/ui2/src/styles/color.rs

@@ -1,5 +1,5 @@
 use gpui::{Hsla, WindowContext};
-use theme2::ActiveTheme;
+use theme::ActiveTheme;
 
 #[derive(Default, PartialEq, Copy, Clone)]
 pub enum Color {