Dark mode for javadocs

[NotMyFault]

Using @media (prefers-color-scheme: dark) we should add a darkmode to the index files of

• https://github.com/IntellectualSites/plotsquared-javadocs
• https://github.com/IntellectualSites/fastasyncworldedit-javadocs
  to increase accessibility for people working in dark mode. As CSS color palette, I recommend using the Jenkins dark mode, as seen on https://weekly.ci.jenkins.io/, for background colors, links etc.

This issue is unrelated to this repository, but javadocs are documentation still, and I thought it's better to file this issue in a centralized space.