Feature hasn't been suggested before.

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Describe the enhancement you want to request

I am proposing the addition of two new built-in themes to OpenCode:

1. Undertale: A high-fidelity theme based on the Undertale battle aesthetic.
2. 2. Deltarune: A neon-themed Dark World aesthetic.
      These themes have already been implemented in PR feat: add Undertale and Deltarune built-in themes #8240.