ui: Allow selecting a different dark/light-theme setting than device setting

[chrisbobbe]

From beta feedback:

The beta is great, especially the responsiveness compared to the old app. Just a minor question - is there a way to turn off dark mode, despite it being my OS setting?

Currently, the app UI follows the device setting for dark/light theme.

This issue is for an in-app setting to choose one or the other for the app, independently of the device setting.

An instance of:

• Store some client settings #97.

[chrisbobbe]

As I noted on #1139:

Storing some client settings is #97, and I see a core team member (@PIG208) has just self-assigned that issue. So let's wait to work on this until he implements a settings UI where this setting will go.

The same applies here.