Dark Mode (#912)

closes #704

app/components/Theme.js

@@ -0,0 +1,25 @@
+// @flow
+import * as React from 'react';
+import { inject, observer } from 'mobx-react';
+import { ThemeProvider } from 'styled-components';
+import { dark, light } from 'shared/styles/theme';
+import GlobalStyles from 'shared/styles/globals';
+import UiStore from 'stores/UiStore';
+
+type Props = {
+  ui: UiStore,
+  children: React.Node,
+};
+
+function Theme({ children, ui }: Props) {
+  return (
+    <ThemeProvider theme={ui.theme === 'dark' ? dark : light} key={ui.theme}>
+      <React.Fragment>
+        <GlobalStyles />
+        {children}
+      </React.Fragment>
+    </ThemeProvider>
+  );
+}
+
+export default inject('ui')(observer(Theme));