Add setting to follow system prefered color-scheme

[Myrstad]

Issue:
Pywalfox has no way to use the prefered theme of the system. Changing prefered theme does update the browsers prefered theme, but has no option of disabling or being set externally. Intended behavour works when disabling pywalfox's theme.

Intended:
Example when using gsettings set org.gnome.desktop.interface color-scheme "prefer-light" pywalfox should use the light theme AND set the browsers prefered theme to light mode, so websites also follow this preference. The current behavour is that light mode is set in a time frame, and there is no way of dynamically set the pywalfox theme AND the firefox preferred theme.

Fix:
Implement a setting that allows pywalfox to follow system theme, or make pywalfox follow the firefox theme. Such as tools like gsetting and the like, can be used to dynamically set the theme.