Getting Qt apps to look good in GTK desktops
 

The Howtos I've seen for how to do this have been a bit... long and unfocused. Here's the TLDR.

Install qt5ct

Optionally install kvantum

Set the following in ~/.profile

Reboot :)

You'll be able to start "Qt5 Settings" and use it to your Qt "Style", fonts and icons. If you've installed Kvantum, then you'll see it in the list of Styles and you can start "Kvantum Manager" to set it up.

I recommend starting with Breeze for both the Style and Icons, and setting the font to be consistent with the GTK desktop. That should look good with most setups.