Fix background color for window controls overlay in titlebar

[mrleemurray]

Resolve the background color issue for the window controls overlay in the titlebar by directly using the theme's agentsBackground. This change ensures the overlay displays the correct color when the theme changes, addressing the transparency issue.

Fixes #312914

[Copilot]

Pull request overview

This PR fixes the Electron Window Controls Overlay (WCO) background color in the Agents Sessions titlebar by resolving an explicit, opaque color from the active theme instead of relying on element.style.backgroundColor (which is intentionally empty due to the transparent/gradient titlebar styling in sessions).

Changes:

• Resolve the WCO background color from the agentsBackground theme token (instead of element.style.backgroundColor).
• Ensure the resolved color is opaque via makeOpaque(WORKBENCH_BACKGROUND(theme)), matching established workbench behavior for WCO/titlebar backgrounds.
• Update WCO style caching/update logic to compare against the resolved bgColor/fgColor values.

[benibenj]

This doesn't fix it.

The issue is the gradient which starts at the top left and goes to the bottom right. You need to correctly compute the color which will apply to the top right. This works by coincidence on macOS because the WCO is in the top left, but does not wok in windows and linux where it is in the top right.

[mrleemurray]

@hawkticehurst will be reducing the gradient size to avoid color clash

[mrleemurray]

Pulled in Hawk's new gradient - please retest @benibenj