Allow theme based on system preferences

[mborgerson]

Description

Many apps are themed based on system settings. It would be nice to support using the user's system-wide color scheme (dark / light) on startup.

Alternatives

No response

Additional context

No response

[zwimer]

Seems useful: https://github.com/albertosottile/darkdetect

[zwimer]

They have a listener class as well that can be a thread target.

[samybtt]

Hello @zwimer, I have a small fix for this (using the "Current" option for the system theme) but I am having troubling rebuilding the project to test it. I've done it yesterday before the PySide6 refactor (#733 ), it was working with PySide2. I'm using python 3.9.5 on ubuntu 20.04.
I can make the pull request if you want to.