Restructure Classic UI / Theming Chapter

[MrTango]

Links of affected pages in Plone Documentation, if any.

• https://6.docs.plone.org/classic-ui/theming/index.html

Description

The sub Chapters like we have them in the trainings, is not help full here. It will lead to not knowing where to put information and having them in more than one place.

I suggest we keep it as flat as possible in this reference documentation.
For now is see chapters like this:

On index page, explain in short the 3 ways of theming.

• Change Logo & Favicon
• Custom CSS TTW
• CSS Variables
  (Custom Properties)
• Creating Theme Packages
  'only the creating part (plonecli), not to much details, reference training for details'

  Custom Barceloneta Theme

  Custom Diazo Theme

  Custom Theme from scatch

• SCSS Structure
• Color modes

[stevepiercy]

@MrTango does this issue supersede #1286, and should the other be closed?

I'll try to get you in Discord to get more details.