Customizing User Manager's Hotspot Login Pages

When a hotspot user wants to buy more time, they get to this page:

https://USER_MANAGER/user/SOMETHING

and login. When they do that, they see this:

Screen Shot 2018-11-26 at 1.37.49 PM.png
Are these customizable at all? I looked in the User Manager’s files/, and the only thing I see is the umfiles folder, which doesn’t appear to contain this html page.
The average user has no idea what a profile is, or know that they need to buy one. I could make these more obvious if I had access to the html file.