Implement "clever"/context-based/"simple" button mode

[rugk]

Background

The typical use case for me may be:

• I am on a light page in the night, now I want it to be dark. Now, when my style currently forces e.g. system-based or a browser-based theme, I just want a dark one with one single click.
• or vice versa: I am on a dark page in the direct sunlight, now I want it to be light (because of better contrast I guess??). Now, when my style currently forces e.g. system-based or a browser-based theme, I just want a light one with one single click.

Currently, based on what settings I have, the one toggle is already set to being. Even with #44 I would need two clicks to change this, which is embarrassing.

Proposed solution

The problem is most of the other settings cannot be used together with that, so we need to hide/disable most of them if that is enabled.
Also, this only makes sense when the add-on itself does not force a light or dark mode (because then clicking on

The UI may switch to this:

[x] Simple mode

Let browser follow…
( ) …system-design
( ) …browser-design

The button then always indicates the current design and reacts on system/browser design changes and

Alternatives

The current way/not offer this?

Additional context

Only possible since v2.0

[rugk]

For everyone following, please see my PR here for a (buggy) draft: #50

However, the technical implementation has some bigger problems I did not consider and maybe it is not possible to implement this at all.

If anyone wants to further pursue it, feel free.