WIP: Add styling ADR

[ljowen]

What this PR does

Propose breaking changes to theme and styling

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.

[CLAassistant]

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