Hi there,
i just switched to RebornOS, hope there may be some help here, too. 
im using cinnamon and i appreciate a common look and feel throughout all applications.
In EndeavourOS virtualbox and qbittorrent for example had the same font and font size as cinnamon applications.
In RebornOS on the other hand, qbittorrent and also virtualbox use larger fonts and this just doesn’t look well.
What am i missing to install so the mentioned applications use the same fonts and font size as my gtk apps?
GUI programs QT are configured using the qt5ct application.
You need to install qt5ct and additionally you can install qt5-styleplugins from the AUR.
and you need to enable to use qt5ct inside /etc/environment:
https://wiki.archlinux.org/index.php/Qt#Configuration_of_Qt5_apps_under_environments_other_than_KDE_Plasma
Or using Kvantum:
community/kvantum-qt5 0.19.0-1 (1016.9 KiB 7.3 MiB) (Installed)
SVG-based theme engine for Qt5 (including config tool and extra themes)
install package and add QT_STYLE_OVERRIDE=kvantum to /etc/environment (systemwide) or ~/.config/environment.d/env.conf per user.
Thnx to both of you, this worked well.