Originally Posted by GraciousMe
Any other suggestions for beginner-friendly CSS-editors?
Honestly, CSS as it applies to ePubs, is basic enough that using a dedicated CSS editor (where you're either going to have to repeatedly import the produced css file, after each change, into the existing Sigil epub, or you're going to have to manually extract the contents of the ePub to work on it) is going to be terribly counter-productive.
There's going to be no easy way to use a dedicated CSS editor in conjunction with a Sigil-built ePub.
A CSS tutorial (like the w3schools link mrmikel provided), and/or examining ePubs that are formatted in a way that you'd like to emulate, is going to do you much more good than any CSS editor will.