Coding the New Opt In forms for Wordpress

Is there a way to access the code for the Infusionsoft Opt In Forms plugin for Wordpress?
There is a blank spot where you can “include custom CSS,” but when I put my code in, the form doesn’t reflect the changes.

I got all the code for the form by inspecting the element, then I added my custom code. It makes the changes I want, but it looks wonky. AND when I drop that code into a code editor, everything looks good. So I know I’m coding everything correctly.

Is there any easier way to code these forms?