feat: add dark mode support

[adnjoo]

can add dark mode?

[lukekarrys]

Yes, this is a great idea! Here are the tasks I see as needed for this:

• Add dark mode toggle to UI
• Use primer/react auto mode to detect system dark mode
• Save dark mode setting to local storage on change
• Audit use of <Theme> components so theme="light" sets it back to the default

[laine-hallot]

@lukekarrys is there a color set defined somewhere that this could be based on? I wouldn't mind making a PR for this