Feature: Color Picker

[herzhenr]

The app uses the Material You design but currently has no method to change the color theme of the app. Issue #11 would allow to match the app on Android to dynamically use the system defined colors but defining a theme manually could be another theming option. This Medium Blog Post shows an example for the primary color selection. This could be extended so a color pallet can be selected which also includes setting the secondary and tertiary colors.