Excel theme black and font list

ZigoZago 

Is that only for fonts or for other gallerias as well? E.g. formats

Hello Zigo Zago. You've got a very interesting problem. Several of us (including a few people on the Excel team) have tried to reproduce it and we can't.

If you go to Excel's Help Tab and Send Feedback. Choose "I don't Like Something". In the comment, put ZigoZago from TechCommunity so they can find your feedback.
Make sure to include an e-mail address so they can ask questions.
The submission will send enough telemetry that they can know your Windows OS and which version of Excel and will help them to get to a solution faster.

In a very strange geek sense... you've done something remarkable - the first to document an odd bug. When you find it and it gets fixed, you can brag about it to your friends and co-workers.

Let us know after you've sent Feedback so the Excel team can find it.

I have no idea what might cause this. Have you tried a repair of Office?