The cell renderer in xfce4-settings-manager/xfce-text-renderer.c should handle the hover effect (as it does in 4.12 / Gtk+2), but this behavior is currently broken in 4.14.
I realized after reading the code that this is connected to a FIXME related to using XfceTextRenderer again. Currently this custom renderer (which provided the hover effects) is not used and if trying to use it as is, it's broken. We should either look into using some more modern widget which also allows filtering (e.g. gtkflowbox seems like an ideal candidate) or really revive the custom renderer.
This bug is tackled in this branch: https://git.xfce.org/users/ochosi/xfce4-settings/log/?h=fix_hover_effect Please test.
Simon Steinbeiss referenced this bugreport in commit 57987f3e8ecee0f95effe0e98d773e517b54c3ee Fix broken hover effect in the settings manager (Bug #14539) https://git.xfce.org/xfce/xfce4-settings/commit?id=57987f3e8ecee0f95effe0e98d773e517b54c3ee
Tested, reviewed and pushed to master.