[BB-2331] [Discovery] Theming React MFAs and non-react systems uniformly

Story

“As an Open edX theme developer, I want a simple forward-compatible theming system so that upgrading a theme from one release to another requires as little effort as possible.”

Full description

Currently, there is no single way to theme both the edx-platform and MFEs in a uniform way that gives a uniform look and feel that can be uniformly customised.

As part of this ticket, break down the requirement for building such a system. The aim is to have a theming system that allows deep customisation while reducing upgrade cost as much as possible.

When looking at deeper customisation consider examples like Apros.

Refs:

• https://openedx.atlassian.net/wiki/spaces/BPL/overview
• https://edx.github.io/paragon/
• https://edx.readthedocs.io/projects/open-edx-release-notes/en/latest/koa.html#developer-experiences

Completion criteria

• Create a discovery document that outlines the comprehensive theming system.
• Create further discovery tickets and dev tasks that can be used here.
• Make sure you coordinate with Filipe from eduNext

[https://tasks.opencraft.com/browse/BB-2331]

Reviewer 1: @bradenm Reviewer 2: @guruprasad