[Plugin: ThickBox] How to style link to inline content?

I’m wondering how to style the link that opens the inline content I’ve implemented in the body text at https://www.arborhealing.com/

I’ve tried adding thickbox.css that I found at https://jquery.com/demo/thickbox/ and it seemingly had no effect on the content (but maybe I did it wrong).

I looked at the plugin files and didn’t see where CSS even existed, but I must be missing it (?) or maybe I need to add it elsewhere?

Is there a way to style inline content and the link pointing to this? The link doesn’t seem to respond to other CSS elements I have in the site for link style.