My primary icon theme and widget style are 20+ years old and not flat in the least. You can still have that look and feel on a real computer if you want it (but you may have to compromise elsewhere or do some extra work). On phones, all bets are off.
Dunno what your issue with that icon pack is, but I’d bet there’s a good chance it can be solved with a few file renames or symlinks if you care enough to bother.
If you’re interested in doing the tech equivalent of a party trick (except that it’s less interesting to watch), go ahead and try. You’ll probably just end up reinstalling almost every package on the system that differs between the base distro and the offshoot. Harmless, but also pointless, since you could just have installed Debian from the get-go and saved yourself a lot of trouble.
There are a whole bunch of Very Silly Things you can do in the Linux world that aren’t worth the effort unless your income relies on the creation of niche Youtube vids. For instance, it should theoretically be possible to convert a system from Debian to Gentoo without wiping and reinstalling. I’m not going to try it.