[feature/frontend] ecks pee theme

[tsmethurst]

Description

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This pull request adds a custom theme modeled after the purely fictional operating system ecks pee, from an alternative 90's/early 00's.

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.