Why doesn't the Custom Fields addon let me change colors or fonts?
The Custom Fields addon lets you add new fields and modify the labels of existing fields. But it won't help you change the look and feel of your form by changing colors or fonts.
Your website's theme controls the styles of elements on the page. And every theme does this differently. There's just too much variety out there for such a plugin to reliably work with every theme — or even most themes. That's why I've chosen not to offer such features.
If you want to adjust the style of your booking form, I'd encourage you to seek out a developer who can write some custom CSS code to make it look great. Or try one of my themes. They're all built with beautiful form integrations.