How to edit CSS when using an html plug-in from another site

Hi, I am very much a beginning w/ CSS but am curious about how to edit the CSS on my website for a section that includes HTML from an external plug-in? Specifically, I have a donation form from Custom Donations that I’ve embedded into my site. Their support says I need to edit the CSS in webflow to change the colors, fonts, etc. to look like the rest of our site.

Thanks!


Here is my site Read-Only: https://sewn.webflow.io/donate
(how to share your site Read-Only link)