Whoever at Beelink is going through the effort to customize the button highlight color in TVSettings and Settings, please resist this urge in the future. It's not adding any value, and now many of us are going through the effort to build custom firmware just to set it back to the Android defaults.
Seriously, the folks at Google have professional graphic design experts who have put a lot of thought into their material design theme. Let's try to keep the stock Android user experience intact.
Apktool allows you to make the modifications to the files within the Settings and TvSettings APKs that I described above. So, if you don't want to update firmware, you could install a root file manager (like CMFileManager or ES File Explorer) and simply copy over the modified APKs from my custom image. (Note: Be sure that you don't change the file permissions in the process.)