Add dimmer css and change dimmer opacity

- This solves a bug where clicking on "Change password"
would make the popup layout incorrectly

ui/src/App.js

@@ -9,12 +9,18 @@ const GlobalStyle = createGlobalStyle`
     height: 100%;
     margin: 0;
   }
+
+  /* Make dimmer lighter */
+  .ui.dimmer {
+    background-color: rgba(0, 0, 0, 0.5);
+  }
 `
 
 import 'semantic-ui-css/components/reset.css'
 import 'semantic-ui-css/components/site.css'
 import 'semantic-ui-css/components/transition.css'
 import 'semantic-ui-css/components/menu.css'
+import 'semantic-ui-css/components/dimmer.css'
 
 class App extends Component {
   render() {