fix #82 by adding system theme in addition to dark and light themes

This MR will add a third option for theme setting: system theme. This allows users to use the globally set theme, rather than only the hardcoded Teleports dark or light theme.

The UI design will replace the switch with three buttons. The selected theme will show a Button of style "highlighted". Unless we want to move this to the settings page, the design options are quite limited due to space. But I like having this option quickly available in the menu, so I would rather not move it to settings page.