Hi @chris26,
You can update the background color from within Branding > Universal Login > Customization Options:
After saving and publishing you should be able to click the Try button and test your changes. Make sure there are no custom pages enabled in Branding > Universal Login > Advanced Options, they will activate the Classic Login and your changes will not be visible.
For the 402 Payment Required error see this post:
If you have any other questions feel free to let us know.
Have a good one,
Vlad