fix: handle malformed custom themes

[KrishavRajSingh]

Issue for this PR

Closes #18517 #12903 #4763

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Fixes the tui crash when the custom theme is malformed. It skips invalid custom theme files instead of crashing, logs the warning and fallbacks to opencode built-in theme.

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How did you verify your code works?

Screen.Recording.2026-03-22.at.6.55.50.AM.mov

If this is a UI change, please include a screenshot or recording.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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[github-actions]

Hey! Your PR title handle malformed custom themes doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.