Refactor theme detection

modules/themeUtils.js

@@ -0,0 +1,20 @@
+"use strict";
+
+export async function detectSystemTheme() {
+    try {
+        const t = await browser.theme.getCurrent();
+        const scheme = t?.properties?.color_scheme;
+        if (scheme === 'dark' || scheme === 'light') {
+            return scheme;
+        }
+        const color = t?.colors?.frame || t?.colors?.toolbar;
+        if (color && /^#/.test(color)) {
+            const r = parseInt(color.slice(1, 3), 16);
+            const g = parseInt(color.slice(3, 5), 16);
+            const b = parseInt(color.slice(5, 7), 16);
+            const lum = (0.299 * r + 0.587 * g + 0.114 * b) / 255;
+            return lum < 0.5 ? 'dark' : 'light';
+        }
+    } catch {}
+    return 'light';
+}