Add configurable syntax tokens for XML/HTML tag highlighting

[CasualDeveloper]

Problem

XML/HTML tags are styled using hardcoded color mappings in getSyntaxRules():

{ scope: ["tag"], style: { foreground: theme.error } },
{ scope: ["tag.attribute"], style: { foreground: theme.syntaxKeyword } },
{ scope: ["tag.delimiter"], style: { foreground: theme.syntaxOperator } },

This means custom themes cannot independently control tag colors—they inherit from unrelated tokens (error, syntaxKeyword, syntaxOperator).

Proposed Solution

Add three new optional theme tokens:

• syntaxTag - for tag names (<div>, <span>)
• syntaxAttribute - for attributes (class=, href=)
• syntaxTagDelimiter - for delimiters (<, >, />)

These would be optional with backward-compatible fallbacks to the current hardcoded values.

Files to Modify

• packages/opencode/src/cli/cmd/tui/context/theme.tsx
• packages/web/public/theme.json (schema)
• packages/opencode/src/cli/cmd/tui/context/theme/opencode.json

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Theming: Unable to change text input placeholder color #5088: Theming: Unable to change text input placeholder color (same pattern of missing dedicated theme tokens)
• [FEATURE]: Separate theming of sidebar background color #5286: Separate theming of sidebar background color (extends theme customization requests)
• [Feature Ask] Hot reload or add keymap to reload the theme #815: Hot reload or add keymap to reload the theme (would complement theme token additions)
• Feat: Improve UI customization / accessibility #2750: Improve UI customization / accessibility (broader theming/customization request)

These issues share the common theme of requesting more granular control over theming and syntax highlighting in OpenCode. Feel free to ignore if your specific case requires something different!

[CasualDeveloper]

Reviewed the related issues flagged by the bot - this is not a duplicate.

• Theming: Unable to change text input placeholder color #5088 - Same pattern (missing theme token) but for placeholder text, not syntax highlighting
• [FEATURE]: Separate theming of sidebar background color #5286 - Requests separate sidebar background token, different scope
• [Feature Ask] Hot reload or add keymap to reload the theme #815 - About hot-reloading themes, unrelated
• Feat: Improve UI customization / accessibility #2750 - Broader accessibility/UI customization, different scope

This issue specifically addresses syntax highlighting tokens for XML/HTML tags.

[CasualDeveloper]

I have a working implementation ready - typechecks pass.

[CasualDeveloper]

Re-committed with valid GPG signature. PR ready: #6130