Something I had forgotten about is that opening some non KDE programs in a KDE desktop leaves me with tiny fonts that don't seem to respond to changes in 'Fonts' in the KDE Settings menu.
One of the most used programs where this is annoying is Synaptic Package Manager and there are numerous threads about it, not one of which provided a solution that worked.
The problem is most annoying on a High Resolution display so here's what I did for my own screen.
MENU > System Settings > Application Appearance >
Style > QtCurve (This was suggested as a fix but didn't work on its own. There's nothing about Fonts in its Configuration option.
Fonts > Force DPI: 150 (96 is the default)
So now this makes all my KDE stuff too big and I choose 'Adjust All Fonts', > Size > and pick something smaller.
Pick a size and check some KDE programs and see the font sizes, then open Synaptic and LibreOffice and check them.
It comes down to a balancing act because LibreOffice and Synaptic use a slightly different method to display.
So far this has been the ONLY simple way I have found to balance the appearance between KDE and soem Non-KDE Programs.