Currently LO's qt5 / kf5 VCL backend just uses the QStyle interface to draw the LO UI to look like native widgets. As a result, this has limits for various UI designs rendered by native Qt style engines not exposed via that interface.
Already a few years ago, Caolan started to introduce the new "weld" theming interface, which uses native - in his case gtk+ - widgets, to get even better GNOME integration.
For the latest progress report, check out his blog: https://caolanm.blogspot.com/2019/10/native-gtk-dialogs-in-libreoffice.html
Currently that API is very gtk+ specific, but this shouldn't prevent adaption by the Qt backend in general.
*** Bug 130758 has been marked as a duplicate of this bug. ***
*** Bug 129293 has been marked as a duplicate of this bug. ***
*** Bug 128372 has been marked as a duplicate of this bug. ***