• As with the frontend, CREATE the following directory structure in your child theme: /[absolute]/[path]/[to]/wp-content/themes/[my-child-theme]/wppizza/css/
• Recommended Way: if you want to customise some admin css declarations, CREATE styles-admin-custom.css in /[absolute]/[path]/[to]/wp-content/themes/[my-child-theme]/wppizza/css/ and add your declarations as required. This file file be read AFTER the main admin css.
• Optional: if you want to actually overwrite ALL declarations, COPY styles-admin.css from wppizza/css to /[absolute]/[path]/[to]/wp-content/themes/[my-child-theme]/wppizza/css/ and overwrite. If you do, only this file will be read. Any future amendments to the css of the plugin itself will not be read, so know what you are doing.
• if you must, you can do both of the above

Again, you could also use your main theme and create the above mentioned directory structure there, but using a child theme is by far the better and recommended way to do this – see WordPress Child Themes)