Since 1.5.6, you’ve been able to override the default PMPro stylesheet.
PMPro now checks for CSS files in a paid-memberships-pro subfolder of your active theme and uses those admin.css, frontend.css, and print.css files instead if they exist.
So to override either of the PMPro stylesheets, just create a folder called “paid-memberships-pro” in your active theme and then copy admin.css, frontend.css, or print.css from the Paid Memberships Pro plugin folder (/css) to this folder. Change it as needed. PMPro will now use your css file instead of the default one.