Add toggle to use browser's preferred theme (#224)

* Add Auto theme that uses browser's preferred color scheme

This will use dark mode automatically if the browser requests it.

* fixup! Add Auto theme that uses browser's preferred color scheme

* Use a toggle to use system theme

src/client/action/settings.js

@@ -1,6 +1,12 @@
 import appDispatcher from '../dispatcher';
 import cons from '../state/cons';
 
+export function toggleSystemTheme() {
+  appDispatcher.dispatch({
+    type: cons.actions.settings.TOGGLE_SYSTEM_THEME,
+  });
+}
+
 export function toggleMarkdown() {
   appDispatcher.dispatch({
     type: cons.actions.settings.TOGGLE_MARKDOWN,