Where Should I Place Custom CSS for Form Pages?

I've looked in the forums but couldn't find a good solution. Where's the best place to add my custom CSS for the Membership form pages? I can't use my theme CSS file -- as all the CSS is overwritten. And adding "!important" to every line is not an option. What do you recommend? I'm assuming I'm not going to want to edit the plugin CSS files because of future updates.

Thank you for your help.