[FEATURE]: Base16 Default Theme for TUI (Dark & Light)

[snelling-a]

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

Summary

Introduce a new theme, base16-default, for the opencode TUI, following the Base16 philosophy and guidelines.
Supports both dark and light variants using official base16 default-dark and base16 default-light palettes.

Details

• Color palette: Uses official base16 default palette for both dark and light modes.
• Philosophy: Follows base16 usage guidelines for mapping all color slots.
• No hardcoded hex values: All theme slots reference palette color keys, ensuring maintainability and consistency.
• Light mode: Inverts base00–base07 for light mode, as per the default palettes.
• Semantic and syntax slots: Appropriately mapped to base08–base0F for variables, errors, warnings, strings, functions, etc.

Motivation

• Provides a canonical, spec-compliant base16 theme for TUI.
• Ensures color consistency and accessibility in both dark and light.
• Makes it easy for users to build or extend base16 themes.

References

• Base16 Styling Guidelines
• Base16 Default Dark YAML
• Base16 Default Light YAML

[github-actions]

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

• [FEATURE]: Add ef-themes collection by Protesilaos Stavrou #5862: [FEATURE]: Add ef-themes collection by Protesilaos Stavrou (another standard theme collection proposal)
• Use terminal theme colors by default #101: Use terminal theme colors by default (related to default theming philosophy)
• Support dark/light mode os sync #480: Support dark/light mode os sync (related to light/dark mode support)

Feel free to ignore if none of these address your specific case.