Integrate external theme functionality into program

[tegridyfarmer]

Problem Statement

Currently there is no built theme option in the program itself.
They have to installed by following the instructions here.
This is not user friendly, since users might be less aware of this feature since it is not in the settings.
Besides installing it requires multiple steps which would be easier to have integrated in the settings.

❔ Possible Solution

Have a setting to change your current theme to one listed in the themes repository.

⤴️ Describe alternatives you've considered

The program could show how to install the themes in the settings, so the users are more aware of the feature.

➕ Additional context

The theme repository: https://github.com/johannesjo/super-productivity-themes