Fix #260: Add Auto theme preference to set theme automatically. (#835)

* Auto theme preference added

* Fix lint

* Add and use AUTO from consts

* Add shared custom hook to get current theme

* Moved up 'Auto' choice

* AUTO -> AUTO_THEME_ID & extract useCurrentTheme to file

* Liberalise theme setting

* Add tests

ui/src/themes/useCurrentTheme.js

@@ -0,0 +1,14 @@
+import { useSelector } from 'react-redux'
+import useMediaQuery from '@material-ui/core/useMediaQuery'
+import themes from './index'
+import { AUTO_THEME_ID } from '../consts'
+
+export default () => {
+  const prefersLightMode = useMediaQuery('(prefers-color-scheme: light)')
+  return useSelector((state) => {
+    if (state.theme === AUTO_THEME_ID) {
+      return prefersLightMode ? themes.LightTheme : themes.DarkTheme
+    }
+    return themes[state.theme] || themes.DarkTheme
+  })
+}