Ok, it’s now an extension! Search WoofWoofCodes/pxt-custom-questions in the extensions search bar, or paste this link: https://github.com/WoofWoofCodes/pxt-custom-questions
This extension lets you change the colors of certain element of the Ask For Number and Ask For String blocks.
Unfortunately, the “background” option doesn’t actually do anything because the original blocks were programmed to have a black background, so actual changing of the background color was never added. Maybe it’s time for me to learn how to make pull requests. I could always just copy the code for the block to change it instead of using the intended theme interface. I’ll update the extension later if I decide to do that.