Guide: developer chapter on styling with CSS and SCSS

[rbeezer]

This chapter was written in response to the following request, from Rob Beezer:

Suppose you are a contributor to PreTeXt. You know some basic CSS principles and tools, but are not an expert or a web developer. (I am such a person). You would like to read something short (1 to 2 pages in a PDF form?) in the Developer's part of the Guide that will help you navigate our SCSS setup. Please develop such material, working on a branch.

Of course, read our CSS code. Color options, theme options, and support for varied HTML-derived outputs may be unique features. There is a README that may be along these lines, and should be consulted. Read or consult whatever else you need externally to understand SCSS generally.

The result is a new chapter, "Styling with CSS and SCSS," in the Developer's Guide, plus a cross-reference to it from the publisher styling section.

Claude Opus 4.7, acting as a coding assistant for Rob Beezer

[rbeezer]

@jjrsylvestre and @ascholerChemeketa - would love your reaction to this one.

@ascholerChemeketa - a single commit on top of all this would be a reat way to make edits. Or we can discuss at Drop-In if more needs to be said.

[ascholerChemeketa]

It's a good start. And would be very good to have. As discussed in breakout, I'll make a PR that assumes this is in place and supplement with a video demonstrating how to use the inspector to reverse engineer where styles are coming from.

[rbeezer]

For the record, @ascholerChemeketa will make a short video, and edit what is here to suit. (Thanks!)