Theme color for the right panel, so it can stand apart from the chat

[pouyarezvani]

Today the right panel (the PR view, the diff, files and the browser) paints with the same theme color as the chat, canvas. There is no way to give it its own color, so the PR view reads as part of the conversation, even with a theme that sets the sidebar apart.

Proposal

Add an optional panel theme color for the whole right side: the tab bar and every view in it. A theme that does not set it falls back to canvas, so existing themes look the same. The theme editor could list it next to Sidebar.

{
  "colors": {
    "canvas": "#101010",
    "panel": "#1a1a1a"
  }
}

The mock-up uses made-up content and follows the current dark layout.