For something that powers the most popular wiki on the internet, making Mediawiki themes is complicated. More complicated than making themes for phpBB, myBB or Wordpress. There is a learning curve.
I used to run a wiki website, but what I had the idea for, evolved into a better idea that would no longer need a wiki, so I coded the website for it from scratch. That's why I don't run wiki websites any more, so I've not had much experience with Mediawiki as a webmaster, only editing Wikipedia.