Menu icon background gets overwritten on hover

[msujew]

Bug Description:

When hovering over a menu entry that has been contributed by a plugin, it's icon background css rule gets overwritten by the changes in #12252.

Note this is browser (or custom frame in Electron) only - we don't support icons in native Electron windows yet.

Steps to Reproduce:

1. Download and install this extension.
2. Open the gutter menu and hover over the menu entry
3. Assert that the icon disappears

Additional Information

• Operating System: Any
• Theia Version: 1.40.0