Very crappy demo (and Im not using topic cards) but I think you can easily do this with just the built-in category background image:
If you just add a category background in the category settings
that places the image on the body tag. Maybe then adjust the CSS to something to your liking (my example is 400px):
`+ some tweaks to text-alignment, you should get an approximation of what you want I think? Main downside is that it’s really full-width so you’ll see it stick out on wider screens.
It’s not perfect, but it’s fast though 