Refactor Dark Plus and add new maintainer

[David-Else]

I have brought #8719 back from the dead, it was confirmed good by the previous theme maintainer @kirawi. It started out as a load of changes that mainly got rejected, and ended up as a really nice refactor making it much more readable.

I have merged in the new changes from master, and attempted to credit @saccarosium using the info from https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors in the commit details (with a temp email that might need updating if it is needed at all).

@the-mikedavis I am not sure if I need to add an accurate Co-authored-by: NAME NAME@EXAMPLE.COM in the commit to credit saccarosium, or maybe the fact I merged his closed PR and see his commits will do this automatically? If I need an email, how do I obtain the right email and do that.

@kirawi You asked me if I wanted to take over maintenance of this theme in the previous thread. I will have a go, cheers!

The only visible difference between the refactor and the previous version I can find is that the whitespace is now much better and more accurate to the original, here is VS Code:

and here is Helix with this PR

It used to be this, much too dark and in your face:

[the-mikedavis]

Since you merged the PR you have all his commits so I would consider co-author unnecessary here.

At some point @pascalkuthe and I would like to flesh out the CONTRIBUTING.md and have a section on this: Co-authored-by is good when you want to take part of someone else's commit or sometimes if you're making a change that follows their instructions to the letter. If you can copy their full commit so that git recognizes them as the author (either via cherry-picking or merging as you have here) that's even better, but I usually don't do that if I make any edits to their commit.

But when merging this I would 'squash & merge' so that you are the author of the commit and make sure that @saccarosium is tagged as Co-authored-by. (Squash + merge just to get rid of the merge commits)