GroupCheck dark theme option

[Dambroda]

I'm a roll20 dark mode user, and the group check output was starting to get hard/jarring for me to read because it assumes you're in light mode:

I decided to copy GroupCheck as a custom script for my game so I could make some half-assed theme changes for myself to make the output legible... and then figured I may as well spend the extra hour to make my changes a persistent setting and see if anyone else wants to use them.

• Stays light mode by default, which should retain the same theme as if there were no changes.
• Toggled via !group-check-config --set [darkmode/lightmode]
• Only changes very specific styles, so this should only really be set if your roll20 is already in darkmode; it still inherits things like general font styles from the base roll20 UI. (Setting the script to darkmode while your roll20 is in lightmode would be almost entirely illegible). It probably wouldn't be too much work to be more thorough in what it styles...

I understand this is a very core script and this might not meet the standards for an update to it, and I wont be at all offended if this gets rejected :)

I have no idea what the API_META.offset stuff does at the top and bottom of the script file, so if that needs to be updated let me know