Ignore the "no GUI" comment, don't know what happened there.
To add a new domain, simply navigate to a page on that domain, click on the icon in the toolbar (or add-on menu) and a simple sidebar opens to select a desired color for the domain.
There's also a Settings button that leads to the Options view in add-ons manager where you can delete (but sadly not change) saved colors.

I'm giving it 5 stars, because it is simple to configure, I have had no problems despite the last update being 2 years ago, and it does what I need it to do.

One problem is tho that when you have multiple windows open, and one of them has a colored domain open, it will color everything. If the windows both have a (different) colored domain open, then it follows the focus.
Although the name says Tabs, the description clearly states that it will set the browser window a custom color, so that was expected

Cleverly thought! I always dread doing something stupid on the production websites, this protects me from myself, thank you Joe! :-)

Smart Add-On, Thank You! :)

UPDATE 2.Sept.2020 ERROR: Cannot save the Color with FF 80.01 on Windows 7 = Push the ''save'' Button jump it back to White. Works flawlessly on FF 68.12 ESR.

The update works great, thanks!