Begin porting Zed1 themes to Zed2 (#3793)

This PR is a first pass at porting the Zed1 themes to Zed2.

For the initial release of Zed2 we'll be shipping just the themes that
existed in Zed1, ported to Zed2. The new themes that were previously
added just in Zed2 have been removed.

Release Notes:

- N/A

crates/workspace2/src/dock.rs

@@ -537,6 +537,7 @@ impl Render for Dock {
 
             div()
                 .flex()
+                .bg(cx.theme().colors().panel_background)
                 .border_color(cx.theme().colors().border)
                 .overflow_hidden()
                 .map(|this| match self.position().axis() {