Improve data model & fix tests

app/src/index.tsx

@@ -1,10 +1,21 @@
 import * as React from "react";
 import * as ReactDOM from "react-dom";
-import { App } from './App'
+
+import App from './App'
+import { MuiThemeProvider, createMuiTheme } from '@material-ui/core/styles';
+
+const theme = createMuiTheme({
+  palette: {
+    type: 'dark', // Switching the dark mode on is a single property value change.
+  },
+  typography: { useNextVariants: true },
+});
 
 declare var document: any
 
 ReactDOM.render(
-    <App />,
+    <MuiThemeProvider theme={theme}>
+      <App />
+    </MuiThemeProvider>,
     document.getElementById("example")
 );