I would pick 1 or two fonts and stay with that. Colours even as well. Just to keep it uniform. And when you're taking the thumbnail image. Try and keep them somewhat uniform if in a controlled environment :)
Hey team, I've got a question for you all. I think this in the right area.
Can your profile picture on YouTube and Channel art have a negative effect of sub grow and click through?
At the moment I have a logo and some other general stuff as the header, should I look to improve these and add...