App: bg-sideBarBackground is never generated, so 8 elements have no background #504

Part of #494 (found while building the hosted player's landing screen).

Problem

bg-sideBarBackground is used in 8 places in packages/app/src (the browser chrome, device frame, snapshot pane, tabs bar, trace player controls, the onboarding screen and two workbench buttons), but packages/app/src/tailwind.css defines no sideBarBackground token, so Tailwind never generates the class and those elements have no background. The built CSS contains no rule for it.

It is mostly hidden because other layers paint behind those elements. It showed on a screen with nothing behind it: the page body follows the OS color scheme rather than the app's theme toggle, so in dark mode on a light OS the screen rendered light text on white.

Proposal

Either add the token (--color-sideBarBackground: var(--vscode-sideBar-background)) or replace the class. Adding it changes how all eight places look, so each needs checking by eye in both themes.