Document color usage

[zanerock]

I'm trying to set up a two-color site with a primary and "callout" color. It seems necessary to dig into the code to even find what the color variables are (I'm using Chrome devtools to list everything) and then you kind of have to guess the effect of changing them. For instance, I'm using --ifm-color-content-secondary for the "callout" color, but it seems the actual intent is for secondary actions (yes?). I'd be happy to work on this, but wanted to see if there was anything existing (like internal notes, or code with comments, etc.) that I could draw from.