[FEATURE]: Add theme support for thinking text

[jdanbrown]

Feature hasn't been suggested before.

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

Describe the enhancement you want to request

It'd be nice to expose theme settings for the colors of thinking text. Currently, it looks like it's just hardcoded as 0.6 alpha times the normal output colors:

• https://github.com/sst/opencode/pull/4610

[github-actions[bot]]

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

• #4542: [FEATURE]: Decrease opacity of thinking summaries - This appears to be requesting the exact same theme control for thinking text appearance/opacity that you're requesting here.
• #4927: [FEATURE]: JSONC support for user themes - Related infrastructure for theme customization that would support this feature.

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

[jdanbrown]

Or maybe just make the 0.6 configurable in the theme? — if too many different themeable colors can show up in thinking text.

[jdanbrown]

Thanks!