Styling membership forms

How can I change the styling of the membership forms AND protect those style changes in future upgrades. The grey blue colors are difficult to read and are jarring in a theme utilizing gold and burgundy tones?

I see the css color options in the button.css but I can’t see where to select a color theme.

I manually made changes in several of the css files (subscriptionform, registration popup, button) etc and I don’t want my client to lose these changes if they need to upgrade the plugin