Add light/dark toggle

[PrimeTimeTran]

As a user

I want to have the ability to switch between light & dark modes
So that I can use a theme which I feel helps me eyes have less strain.

Acceptance Criteria

• The theme is set to device theme by default.
• A toggle/switch exists in the app which switches between light and dark mode on press.
• The chosen theme persists between app reloads.

[PrimeTimeTran]

Finished with this commit