It happened a couple of days ago. Clearing the cache did not help. This is what I see now:
This happens with other Stack Exchange sites also. Meta Stack Overflow, on the other hand, looks fine:
I configured the CDN incorrectly in one spot. I'm not entirely sure how or why this caused this issue – my theory at the moment is that the Android browser and the CDN outsmarted each other with regards to caching – but either way it should be fixed now.