Implement light mode

[matr1x-hackmud]

Problem

Works towards #542

Context

Re-enables the default Docusaurus light / dark toggle and adds a hackmuddy light mode that does the following:

• Removes the scan-lines (does so with some color variable trickery)
• Puts a black text shadow around color tags to make sure the colors still stand out on a white background (tweaks welcome)

Screencaps

In accessible mode:

Color shadowing:

[seanmakesgames]

Can you create an issue for this? Why do we need a light mode? What's the problem we're trying to solve?
(Adding light mode means that difficulty of visual design doubles, doubling assets or picking styles that work in both, or all of the above)