Provide a way to manage different themes in Glowing Bear

* The only theme we have right now is ther dark theme
* The dark theme may be hard to use on mobile. In that regard, we have received requests for a light theme.
* This PR provides a way to make it easier for users to create their own theme for GB.
Again this is probably stupid but why are you using `""` for one string and `''` for the other?