On Ubuntu gtk-theme-common snap sets the choosen theme for most of the distros, i was snapping my qt app last night, tested it on ubuntu and got no theming issue.
but when i installed my snap on Elementary Os my app was using fallback cursor theme and not the one used by elementary OS.
I worked out a solution for this -
installing dmz-cursor-theme package on local system made my app use that cursor
sudo apt install dmz-cursor-theme
Also , i noticed my qt app using bigger fonts and not following font size used by default desktop , in that case you need to force your qt app to use gtk style. or if you want your qt app to follow gtk theme, execute app with -
QT_STYLE_OVERRIDE=gtk appname
i hope this will help some one who having these weird issues with snap apps on distributions othen than Ubuntu. thanks