[UI] Add css variables to the brand.css file

Problem/Motivation

If you want a brand color to your element from the theme settings you should add the class or ID or HTML tag to the brand.css file and set a color variable to the current element.

Steps to reproduce

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Proposed resolution

Add CSS variables to the brand.css file.

Remaining tasks

N/A

User interface changes

You can use CSS variables from the brand.css file to set the brand color for your element on the general .scss files.

API changes

N/A

Data model changes

N/A