This is related to QGtkStyle being removed from Qt5.7+. Therefore for now Qt5 apps effectively don't use the Ubuntu native style anymore and such result is expected to happen on Ubuntu for Qt5 apps. There are some options (workarounds) available (basically by continuing to use QGtkStyle) but the end user has to install enable them on their own. No official solution yet. Or i guess if the end user is using a custom FreeCAD stylesheet. That will determine the FreeCAD look. Regardless of the desktop environment used.
Not that i know of. At least not directly. ATM i don't have Ubuntu 18.04 install but i guess it is time to have one (at least in VM) to test things easily. For now i tested only on my local Py3/Qt5 build (different version of Py3 and Qt5) and can't reproduce the issue. I suspect that this could be related:
https://forum.freecadweb.org/viewtopic. ... 20#p250040
And this PR might have fixed things:
As sip is to PyQt what shiboken is to PySide. PySide trying to use sip therefore doesn't make much sense. I am guessing best if first @kkremitzki removes the workaround used and just updates the freecad-python3 package using the latest FreeCAD code. Hopefully the issue will get resolved by doing that. If not i can take a further look on what might be causing the mentioned issue.