New: Dark theme for login screen

(cherry picked from commit cae134ec7b331d1c906343716472f3d043614b2c)

frontend/src/App/ApplyTheme.tsx

@@ -0,0 +1,37 @@
+import React, { Fragment, ReactNode, useCallback, useEffect } from 'react';
+import { useSelector } from 'react-redux';
+import { createSelector } from 'reselect';
+import themes from 'Styles/Themes';
+import AppState from './State/AppState';
+
+interface ApplyThemeProps {
+  children: ReactNode;
+}
+
+function createThemeSelector() {
+  return createSelector(
+    (state: AppState) => state.settings.ui.item.theme || window.Prowlarr.theme,
+    (theme) => {
+      return theme;
+    }
+  );
+}
+
+function ApplyTheme({ children }: ApplyThemeProps) {
+  const theme = useSelector(createThemeSelector());
+
+  const updateCSSVariables = useCallback(() => {
+    Object.entries(themes[theme]).forEach(([key, value]) => {
+      document.documentElement.style.setProperty(`--${key}`, value);
+    });
+  }, [theme]);
+
+  // On Component Mount and Component Update
+  useEffect(() => {
+    updateCSSVariables();
+  }, [updateCSSVariables, theme]);
+
+  return <Fragment>{children}</Fragment>;
+}
+
+export default ApplyTheme;