Tidy layout, introduce styleGuideMode

@nicol I started looking at the 'how to introduce the right css file' problem, then got sidelined a bit.

This merge request:

• introduces Style Guide Mode: in this mode we only show recommended snippets. (re chat discussion)
• improves the layout and markup 😊 remove h3 under h1 which is unnecessary and also not semantic; colour code good and bad stuff.
• some CSS tweaks: simplifying multiple rules with a pattern; moving css var declarations to :root for easier overriding; making some rules more specific so as not to affect he style of the demo html.
• some renaming of js variables for clarity
• 💣 I've removed (just commented for now) the <ul> listing the snippet files per cateogory. This made the nav quite unusuable (forever scrolling), and I don't think it brought much benefit. So it's now a simple, clickable list of categories.