(This page is work in progress. Very much so.)
Maemo Leste ships with a Qt5 port as of mid 2020.
This page documents how some of that works, and some tricks to modify the setup.
Using a non-maemo theme
Unset or override the "QT_STYLE_OVERRIDE" and "QT_QPA_PLATFORM" environment variables.
You can also add the flag "-style=fusion" to set the style.
Menus are not working
Add the flag "-platform xcb". This enforces the libxcb bindings https://xcb.freedesktop.org/ and may help.