New: Native Theme Engine

frontend/src/App/App.js

@@ -4,6 +4,7 @@ import React from 'react';
 import DocumentTitle from 'react-document-title';
 import { Provider } from 'react-redux';
 import PageConnector from 'Components/Page/PageConnector';
+import ApplyTheme from './ApplyTheme';
 import AppRoutes from './AppRoutes';
 
 function App({ store, history }) {
@@ -11,9 +12,11 @@ function App({ store, history }) {
     <DocumentTitle title={window.Prowlarr.instanceName}>
       <Provider store={store}>
         <ConnectedRouter history={history}>
-          <PageConnector>
-            <AppRoutes app={App} />
-          </PageConnector>
+          <ApplyTheme>
+            <PageConnector>
+              <AppRoutes app={App} />
+            </PageConnector>
+          </ApplyTheme>
         </ConnectedRouter>
       </Provider>
     </DocumentTitle>

frontend/src/App/ApplyTheme.js

@@ -0,0 +1,49 @@
+import PropTypes from 'prop-types';
+import React, { Fragment, useEffect } from 'react';
+import { connect } from 'react-redux';
+import { createSelector } from 'reselect';
+import themes from 'Styles/Themes';
+
+function createMapStateToProps() {
+  return createSelector(
+    (state) => state.settings.ui.item.theme || window.Prowlarr.theme,
+    (
+      theme
+    ) => {
+      return {
+        theme
+      };
+    }
+  );
+}
+
+function ApplyTheme({ theme, children }) {
+  // Update the CSS Variables
+  function updateCSSVariables() {
+    const arrayOfVariableKeys = Object.keys(themes[theme]);
+    const arrayOfVariableValues = Object.values(themes[theme]);
+
+    // Loop through each array key and set the CSS Variables
+    arrayOfVariableKeys.forEach((cssVariableKey, index) => {
+      // Based on our snippet from MDN
+      document.documentElement.style.setProperty(
+        `--${cssVariableKey}`,
+        arrayOfVariableValues[index]
+      );
+    });
+  }
+
+  // On Component Mount and Component Update
+  useEffect(() => {
+    updateCSSVariables(theme);
+  }, [theme]);
+
+  return <Fragment>{children}</Fragment>;
+}
+
+ApplyTheme.propTypes = {
+  theme: PropTypes.string.isRequired,
+  children: PropTypes.object.isRequired
+};
+
+export default connect(createMapStateToProps)(ApplyTheme);