app: Session scrollbar is hard to see on some themes

[alexyaroshuk]

Description

It is difficult to see the session scrollbar due to bad contrast on dark theme setting:

OC-1 theme: bad

Ayu theme: bad

Dracula theme: ok

Plugins

n/a

OpenCode version

1.1.48

Steps to reproduce

1. start a session
2. write some prompts
3. observe that scrollbar is difficult to see

Screenshot and/or share link

No response

Operating System

windows 10

Terminal

No response

[github-actions]

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

• Scrollbar is tiny on initial session load; resizing window fixes it #5605: Scrollbar is tiny on initial session load; resizing window fixes it - addresses scrollbar visibility problems
• Text selection has poor contrast/readability on light terminals with certain themes #5295: Text selection has poor contrast/readability on light terminals with certain themes - related contrast/visibility issue affecting UI elements
• Tooltips are unreadable in light mode #9333: Tooltips are unreadable in light mode - similar theme contrast issue

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