TUI Sidebar Light Mode support

[alastairlundy]

Pre-flight checks

• I searched existing issues and did not find a duplicate request.
• I reviewed current behavior on the production released OpenCode version.

OpenCode version reviewed

2.0.21

Problem statement

The plugin's TUI sidebar is hard to see when using OpenCode in light mode.

Proposed change

Use different colours such as Black instead of Light gray if using OpenCode in light mode.

Alternatives considered

No response

Acceptance criteria

No response

[slkiser]

@alastairlundy Thanks for the report, sorry that's a regression I think