[POC][Theming] css custom properties

[LukasHirt]

Description

POC of css custom properties with ponyfill.

Related Issue

• Refs Runtime Theming #2362

Motivation and Context

Enable runtime theming

How Has This Been Tested?

• test environment:
• test case 1:
• test case 2:
• ...

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:

Open tasks:

• Try a different approach to load the theme
• Test with extension
• Test with web component
• Test in IE11
• Test with web component in IE11

[LukasHirt]

Potential limitation - Ponyfill for ie11 is watching for changes only

when a <link> or <style> element is added, removed, or has its disabled or href attribute modified

Ref - https://jhildenbiddle.github.io/css-vars-ponyfill/#/?id=watch