feat: support ".login-panel-dark" CSS for signup/login pages (#3269)

* feat: add custom dark mode CSS for login and registration forms.

* refactor: extract dark theme check to Setting.js

web/src/Setting.js

@@ -1557,3 +1557,7 @@ export function getCurrencyText(product) {
     return "(Unknown currency)";
   }
 }
+
+export function isDarkTheme(themeAlgorithm) {
+  return themeAlgorithm && themeAlgorithm.includes("dark");
+}