Anyone know if there is a way to change the background fill of text cards in Management Reporting?
Short Answer, No!
Don't know if it's possible in text card settings but you can achieve it by publishing a colored image (even just a monocolored square) and set it as background to your text.
Also you can change background color of the full page.
You can also create an image with whatever 'text' you want and use an image card, assuming your text does not need to change.
Feel free to add this idea to the idea exchange.