A: Hi!

You can create a request for Bulgarian in our roadmap. Depending on the demand we will be able to prioritise the addition of this language - https://updates.meiro.cc/roadmap

As for the light mode, while you can't switch to it in the admin zone, you can customise the design of your challenge to match your brand guidelines - https://help.meiro.cc/en/articles/17-challenge-blocks (look for Design in the right menu)