feat: compute scope intersection for theme pair

src/themes.rs

@@ -1,5 +1,5 @@
 use std::{
-    collections::{HashMap, HashSet},
+    collections::{BTreeSet, HashMap, HashSet},
     error::Error,
     fs,
     path::Path,
@@ -218,6 +218,21 @@ pub fn init(config: &AppConfig) {
     }
 }
 
+/// The intersection of scopes between two themes.
+pub fn shared_scopes(light: &str, dark: &str) -> Option<BTreeSet<String>> {
+    let registry = REGISTRY.get()?;
+    let light = registry.themes.get(light)?;
+    let dark = registry.themes.get(dark)?;
+    Some(
+        light
+            .styles
+            .keys()
+            .filter(|k| dark.styles.contains_key(*k))
+            .cloned()
+            .collect(),
+    )
+}
+
 pub fn theme_css_func() -> Func {
     theme_css::func()
 }