Replace color module with a CSS-Variables based module to provide configurable theme parameters

Problem/Motivation

Color module is unmaintainable and outdated. Its functionality OTOH is crucial for the long tail of very-low-budget projects and the adoption of drupal in this segment, making it important to provide this in core.

Proposed resolution

I made a POC: CSS Variables → .

1. Theme author can provide a definition configurable parameters (any property, not only colors) via config schema.
2. If the module is not enabled, the theme-provided defaults are used.
3. If enabled, CSS variables with a stronger selector are autogenerated from config.
4. An optional adapter makes CSS Variable config schema for every legacy Color-enabled theme. Also it helps migrate to CSS Variables
5. It contains a IE polyfill

Let's make this the successor of Color module.

Remaining tasks

• #3090875: Add a server side css variables replacer (for JS-less IE11) →
• #3090874: Add widgets (color, distance, ...) to the configelement autogenerated form →
• #3090884: Create a color2cssvars command to automigrate themes →

User interface changes

API changes

Data model changes

Release notes snippet