This is in iFrame, afaik it is not possible to change that inside Webflow with custom code, at least not with pure css. This is not a problem with Webflow, it is technically not possible to change CSS within an iframe.
I see that you are using a tool called “Now Book it”. I had a quick look at their website, but I can’t find any documentation. Maybe it is possible to add custom CSS to this tool. If it allows to insert custom code, this selector should do the trick: