Header cards place white text on a background. The text automatically changes color on solid backgrounds as necessary, but doesn’t seem to respond at all to photo backgrounds. But obviously, photo backgrounds look better.
Solid white text doesn’t really look great on photo backgrounds. And it’s not especially legible.
The solution I’ve found is to put a tiny drop shadow on the text. I’m sure I could do this by editing my theme, but I want to recommend it so everybody else can appreciate my suggestion. A couple of pixels of black drop shadow on white text makes it way more legible and just prettier. You could also experiment with a black border around white text, or something like that.
I don’t think it’s necessary to add complex text editing options to header cards—I appreciate the relative minimalism of the interface—but I think this one change would make them look way better.