[BUG] VS Code extension webview background hardcoded black, ignores editor theme

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The sidebar webview background changed from using the VS Code theme's background color to a hardcoded near-black/black. Before a recent release the panel background matched the editor theme (dark grey, ~#1e1e1e with my theme), now it renders pure black regardless of theme. This makes it visually inconsistent with the rest of VS Code's UI.

Before:
<img width="908" height="1051" alt="Image" src="https://github.com/user-attachments/assets/7d69edc3-5f34-420a-89e9-81b2f2fad139" />

After / current regression:
<img width="862" height="1052" alt="Image" src="https://github.com/user-attachments/assets/f4e2bef6-fc96-47ee-a77c-9ede8ca70890" />

Platform

VS Code extension (sidebar/webview)

Operating System

macOS 26.3

What Should Happen?

The webview background should use var(--vscode-sideBar-background) so it adapts to the active VS Code color theme, as it did in previous versions.

Error Messages/Logs

Steps to Reproduce

1. Install the Claude Code VS Code extension (latest version)
2. Open Claude Code in the sidebar
3. Observe the webview background is pure black when a chat is started
4. Compare to VS Code's sidebar/editor background — they no longer match

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.58

Claude Code Version

2.1.63

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_