Removing the font

I installed Simple Lightbox and it works well. But the font choice on the lightbox text completely clashes with the font choice on my own website’s branding. And other than hacking the plugin CSS files, I can’t find a simple, easy way to do this. I have tried following the instructions on creating a bespoke theme, but despite being an experienced developer, I can’t get it to work as it doesn’t use my new CSS file. I can see it loaded, but it’s not used. Besides, this feels like complete overkill when all I want is to remove an inappropriate default font on a custom theme!