Parameterize theme2::init to allow loading just the base theme (#3345)

This PR adds a parameter to the `theme2::init` method to indicate what
the theme-loading behavior should be.

This allows us to indicate when we want to load all of the additional
built-in user themes (like in the Zed binary and in the storybook), and
when we don't want to load the user themes (like in tests).

We're using an enum over just a `bool` here for clarity at the call
site.

Release Notes:

- N/A

crates/theme2/src/registry.rs

@@ -100,6 +100,11 @@ impl ThemeRegistry {
             .ok_or_else(|| anyhow!("theme not found: {}", name))
             .cloned()
     }
+
+    pub fn load_user_themes(&mut self) {
+        #[cfg(not(feature = "importing-themes"))]
+        self.insert_user_theme_familes(crate::all_user_themes());
+    }
 }
 
 impl Default for ThemeRegistry {
@@ -110,9 +115,6 @@ impl Default for ThemeRegistry {
 
         this.insert_theme_families([one_family()]);
 
-        #[cfg(not(feature = "importing-themes"))]
-        this.insert_user_theme_familes(crate::all_user_themes());
-
         this
     }
 }

crates/theme2/src/settings.rs

@@ -117,7 +117,7 @@ impl settings::Settings for ThemeSettings {
         user_values: &[&Self::FileContent],
         cx: &mut AppContext,
     ) -> Result<Self> {
-        let themes = cx.default_global::<Arc<ThemeRegistry>>();
+        let themes = cx.default_global::<ThemeRegistry>();
 
         let mut this = Self {
             ui_font_size: defaults.ui_font_size.unwrap_or(16.).into(),

crates/theme2/src/theme2.rs

@@ -31,8 +31,25 @@ pub enum Appearance {
     Dark,
 }
 
-pub fn init(cx: &mut AppContext) {
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum LoadThemes {
+    /// Only load the base theme.
+    ///
+    /// No user themes will be loaded.
+    JustBase,
+
+    /// Load all of the built-in themes.
+    All,
+}
+
+pub fn init(themes_to_load: LoadThemes, cx: &mut AppContext) {
     cx.set_global(ThemeRegistry::default());
+
+    match themes_to_load {
+        LoadThemes::JustBase => (),
+        LoadThemes::All => cx.global_mut::<ThemeRegistry>().load_user_themes(),
+    }
+
     ThemeSettings::register(cx);
 }