Fix/13608 wrong hover color in query select dropdown for certain themes

[``* `#13608` Wrong hover color in query selection dropdown for certain themes``](https://community.openproject.org/work_packages/13608)
Without knowing what exactly happens here, including the same mixin at so many levels in a relatively specific class (`dropdown-scrollable`) somehow tells me there's something wrong with the markup or the use of CSS classes. This is way too complicated.

That this whole thing apparently works with a typo (`.first` instead of `:first`), only confirms that for me.