How to style/theme the unsubscribe form

Sorry if this is a totally noob question but I’ve not been able to figure out how to get rid of the bright purple coloring of the unsubscribe form. Example: https://marketing.sooperthemes.com/email/unsubscribe/5be192a790f84845028500

I went into settings/themes and tried changing the theme. Besides not even getting any indication that I had switched themes, changing the theme does not appear to affect the unsubscribe form. Where does it’s styling come from and how do I change it? I hope it’s not something we’re all stuck with.