I published an article on the Mailcoach blog explaining the setup around customizable themes for newsletter archives.
I relied on CSS custom properties and HSL colors to allow users to customize their newsletter archives without fiddling with too many options.
Colors are often defined in RGB, but RGB is difficult to transform without custom functions. We ask the user to define the primary color in HSL instead. The benefit of HSL is that it's easy to control a color by tweaking the parameters.