Fix knockout icon background to match hovered entry background in project panel (#22258)

No issue attached, as this is something trivial and should have been
part of my merged PR
[here](https://github.com/zed-industries/zed/pull/22073).

For context, in the above-mentioned PR, I removed the hover color from
selected/marked entries in the Project Panel. The reasoning behind this
change is already explained in that PR, but to reiterate:

> This change was inspired by the behavior in VSCode, the Firefox
sidebar, and Dolphin (KDE File Manager). When an item is selected, it
doesn’t display a separate hover state to avoid confusing users about
whether the item is selected or merely hovered over.

@nilskch mentioned in the comments of the above PR that I should have
also changed the background of the knockout icon, which appears on
entries, to match the entry background color on hover. This PR addresses
that.

Before:

![Screenshot_20241219_230005](https://github.com/user-attachments/assets/fd517e01-1a1e-4e63-9320-ba24bf3d4c64)

After:

![Screenshot_20241219_225853](https://github.com/user-attachments/assets/35fb146f-83e4-409b-8061-0efbdbba5cf9)

Release Notes:

- N/A