Try this:
1. Exit Steam
2. Hold down your [Windows] and [R] keys and type this in -> %localappdata%
3. Open up the Steam folder within, delete the htmlcache folder
4. Hold down [Windows] and [R] keys and this time put this in -> steam://flushconfig
See if that fixes it for you.

didnt work

hm, you could try switching to Google's DNS, that's fixed things for some people, as this could be your ISP's fault

https://developers.google.com/speed/public-dns/docs/using#windows

just as a warning read the directions carefully, and if things end up not working or breaking follow the steps at the bottom to "Switch back to your old DNS settings"

css blocked kappa

Update:
We still haven't fixed it entirely,

1. clearing the cache, whether through manually deleting the folder as I described above, or through the steam settings ui, didn't work
2. changing his DNS to Google's didn't do anything
3. adding steam to Windows Defender exclusions list made the background of steam show up black, but the majority of formatting is still not working
4. we made sure that Steam was allowed under his firewall rules too.

at this point I have no other ideas and just wanna chalk it up to his isp (xfinity) being ass about this somehow

if anyone else wants to try their hand at googling this problem, I was looking along the lines of "steam css broken" and stuff