Allow theme based on system

[samybtt]

By setting the theme to "Current", the theme of the system will be used.
This solution uses darkdetect.
Should be linked to #780 and #805 .

[zwimer]

Thanks for contributing :)
This may be a duplicate of #806
Also, it looks like this PR might be lacking auto-tracking of system default; that is automatic switching of themes when users toggle dark mode. Also, as per #805 (comment) "Current" does not mean 'track System'.

[zwimer]

Closing in favor of #806

[ltfish]

@samybtt Thank you for submitting your first PR to angr management! Unfortunately @zwimer has already implemented this feature in another PR, as you can see. Looking forward to your next PR!