add: custom themes

[nanvenomous]

• [x] I have read CONTRIBUTING.md.
• [ ] I have created a discussion that was approved by a maintainer (for new features).

here is a look at my current theme spaceduck:

You can see example config here https://github.com/charmbracelet/crush/pull/1392/files#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5

[charmcli]

All contributors have signed the CLA ✍️ ✅
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[nanvenomous]

I have read the Contributor License Agreement (CLA) and hereby sign the CLA.