Feature Request: Add Undertale and Deltarune built-in themes

[cmdr-chara]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

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.

[github-actions]

This issue is related to #5862, which is a feature request to add the ef-themes collection. Both are requests to add new built-in themes to OpenCode. You may want to check if these could be implemented together or as part of a broader theme expansion effort.