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/layout/Layout.js

@@ -6,14 +6,14 @@ import { HotKeys } from 'react-hotkeys'
 import Menu from './Menu'
 import AppBar from './AppBar'
 import Notification from './Notification'
-import themes from '../themes'
+import useCurrentTheme from '../themes/useCurrentTheme'
 
 const useStyles = makeStyles({
   root: { paddingBottom: (props) => (props.addPadding ? '80px' : 0) },
 })
 
 export default (props) => {
-  const theme = useSelector((state) => themes[state.theme] || themes.DarkTheme)
+  const theme = useCurrentTheme()
   const queue = useSelector((state) => state.queue)
   const classes = useStyles({ addPadding: queue.queue.length > 0 })
   const dispatch = useDispatch()