[docs][themes] Add visual aids to explain themes concept better in docs

[prestonso]

Per conversation with @amberleyromo last week!

It would be really strong to have some visuals to support our theme docs:

• Brainstorming how we can better visually explain the concept of themes, taking inspiration from the visuals associated with Trevor’s case study.
• Designing and implementing these visual aids in the docs.

This is a relatively low-priority issue, but it could also be picked up by the community. This is a minor Learning improvement but is important due to our push for greater Themes adoption.