Custom web interface

I want to have some custom CSS on my Pi-hole’s web interface, but i’m not sure what directory the web interface is inside of, so where is the web interface’s code stored on my raspberry pi?

It’s in /var/www/html/admin/ but any changes you make will prevent further updates from working.

By further updates not working. do you mean updates to the Pi-Hole software or do you mean changing settings for my PI-Hole system such as adding sites to the blocklist?

Updates to the software.