Follow-up: https://github.com/zed-industries/zed/pull/34655  

We should use an opaque fallback color for `panel.overlay_hover`. This
helps when a custom theme doesn’t provide it, nor `element.hover`. For
example, VSCode’s default modern dark theme doesn’t include an
`element.hover` color after import.

Release Notes:

- Fixed an issue where the project panel’s sticky entry could appear
transparent on hover with certain themes.