You now can! Form theming options are now available for paid ChapterBuilder accounts. You can now go to Forms > Edit Form and then on the right hand "Form Options" panel, There you will see "Form Theming." Simply toggle it on and there you will see 3 different color options. 

You can either play around with color or paste in a hex code. 

Most important; don't forget to click "Save" to keep your changes.