Add ui_font_size setting (#3199)

This PR adds a new `ui_font_size` setting that can be used to control
the scale of the entire UI.

We use the value in this setting to set the base rem size of the window.

Release Notes:

- N/A

crates/theme2/src/settings.rs

@@ -17,6 +17,7 @@ const MIN_LINE_HEIGHT: f32 = 1.0;
 
 #[derive(Clone)]
 pub struct ThemeSettings {
+    pub ui_font_size: Pixels,
     pub buffer_font: Font,
     pub buffer_font_size: Pixels,
     pub buffer_line_height: BufferLineHeight,
@@ -28,6 +29,8 @@ pub struct AdjustedBufferFontSize(Option<Pixels>);
 
 #[derive(Clone, Debug, Default, Serialize, Deserialize, JsonSchema)]
 pub struct ThemeSettingsContent {
+    #[serde(default)]
+    pub ui_font_size: Option<f32>,
     #[serde(default)]
     pub buffer_font_family: Option<String>,
     #[serde(default)]
@@ -115,6 +118,7 @@ impl settings2::Settings for ThemeSettings {
         let themes = cx.default_global::<Arc<ThemeRegistry>>();
 
         let mut this = Self {
+            ui_font_size: defaults.ui_font_size.unwrap_or(16.).into(),
             buffer_font: Font {
                 family: defaults.buffer_font_family.clone().unwrap().into(),
                 features: defaults.buffer_font_features.clone().unwrap(),
@@ -123,9 +127,10 @@ impl settings2::Settings for ThemeSettings {
             },
             buffer_font_size: defaults.buffer_font_size.unwrap().into(),
             buffer_line_height: defaults.buffer_line_height.unwrap(),
-            active_theme: themes.get("Zed Pro Moonlight").unwrap(),
-            // todo!(Read the theme name from the settings)
-            // active_theme: themes.get(defaults.theme.as_ref().unwrap()).unwrap(),
+            active_theme: themes
+                .get(defaults.theme.as_ref().unwrap())
+                .or(themes.get("Zed Pro Moonlight"))
+                .unwrap(),
         };
 
         for value in user_values.into_iter().copied().cloned() {
@@ -142,6 +147,7 @@ impl settings2::Settings for ThemeSettings {
                 }
             }
 
+            merge(&mut this.ui_font_size, value.ui_font_size.map(Into::into));
             merge(
                 &mut this.buffer_font_size,
                 value.buffer_font_size.map(Into::into),