I want a , but of course, the background in nightmode will appear black (or something close to that). How can I set the css so the big 7 has the same font-color as the background no matter what the user has set
Let's analyze this question a little bit.
The 7; is it a background image or using some advanced css trick like this? https://stackoverflow.com/questions/15986216/how-can-i-add-a-large-faded-text-background-via-css
I haven't studied the issue in depth, but ebook reading systems generally don't support background images for pages. They generally are behind the curve of css which is supported in web browsers today. Maybe in the next generation of e-readers (5-10 years from now), this will become possible.
If you used a Fixed Layout Book format (on kindle or Apple), maybe you might achieve this effect, but doing so would use a fixed display size and generally disable user controls.
You're asking for the moon, bro....