just copy wppizza-custom.css to your (child)theme directory and only overwrite the styles you need to override.
and
However, if the theme you are using already provides an option somewhere to enter your custom css, you could instead also enter your css declarations there
nowhere does it say anything about wppizza-responsive.custom.css…?!
I cannot make it any simpler than that really and if you never worked with css, then I guess it’s time to google some tutorials if you want to do this. I cannot provide support for the basics of webdesign, sorry.