Rework third-party themes (#3254)

This PR reworks the way we define our third-party themes to make them
work as overlays on top of a base theme.

We introduce the concept of a `UserThemeFamily` that contains
`UserTheme`s. Rather than being an entire theme definition on their own,
a `UserTheme` just contains optional overrides for the values in a
`Theme`.

When resolving a `UserTheme`, we apply it on top of the base theme. Any
values not overridden in the `UserTheme` will fall back to the `Theme`
defaults.

Right now we are just using `UserTheme` to model third-party themes that
we distribute with the Zed binary. However, this same structure can also
be used to import arbitrary user themes (such as from a theme registry,
or even a theme blob from the settings file).

Release Notes:

- N/A

crates/theme2/src/registry.rs

@@ -1,7 +1,14 @@
-use crate::{zed_pro_family, Theme, ThemeFamily};
+use std::collections::HashMap;
+use std::sync::Arc;
+
 use anyhow::{anyhow, Result};
 use gpui::SharedString;
-use std::{collections::HashMap, sync::Arc};
+use refineable::Refineable;
+
+use crate::{
+    zed_pro_family, Appearance, GitStatusColors, PlayerColors, StatusColors, SyntaxTheme,
+    SystemColors, Theme, ThemeColors, ThemeFamily, ThemeStyles, UserTheme, UserThemeFamily,
+};
 
 pub struct ThemeRegistry {
     themes: HashMap<SharedString, Arc<Theme>>,
@@ -20,6 +27,37 @@ impl ThemeRegistry {
         }
     }
 
+    fn insert_user_theme_familes(&mut self, families: impl IntoIterator<Item = UserThemeFamily>) {
+        for family in families.into_iter() {
+            self.insert_user_themes(family.themes);
+        }
+    }
+
+    fn insert_user_themes(&mut self, themes: impl IntoIterator<Item = UserTheme>) {
+        self.insert_themes(themes.into_iter().map(|user_theme| {
+            let mut theme_colors = match user_theme.appearance {
+                Appearance::Light => ThemeColors::default_light(),
+                Appearance::Dark => ThemeColors::default_dark(),
+            };
+
+            theme_colors.refine(&user_theme.styles.colors);
+
+            Theme {
+                id: uuid::Uuid::new_v4().to_string(),
+                name: user_theme.name.into(),
+                appearance: user_theme.appearance,
+                styles: ThemeStyles {
+                    system: SystemColors::default(),
+                    colors: theme_colors,
+                    status: StatusColors::default(),
+                    git: GitStatusColors::default(),
+                    player: PlayerColors::default(),
+                    syntax: SyntaxTheme::default_dark(),
+                },
+            }
+        }));
+    }
+
     pub fn list_names(&self, _staff: bool) -> impl Iterator<Item = SharedString> + '_ {
         self.themes.keys().cloned()
     }
@@ -43,7 +81,7 @@ impl Default for ThemeRegistry {
         };
 
         this.insert_theme_families([zed_pro_family()]);
-        this.insert_theme_families(crate::all_imported_themes());
+        this.insert_user_theme_familes(crate::all_imported_themes());
 
         this
     }