Please take no offense but it looks like in the early nineties. Please breezefy it.
Some people prefer vintage! 😜
That said, since it's that old, should we consider taking a ground-up approach? I'm currently on my work machine (macOS) but I know what pain-points you're referring in the experience of the application that'd help with aligning with Breeze (or even Kirigami standards.
This isn't really something that we can feasibly fix in Kubuntu. I would say, "please report upstream," but KDE folks are well aware of the issue already. What's needed is not more bug reports, but rather someone to actually put in the time to improve it. Would you like to have a go at it?
Bugzilla tickets aren't necessary to commence work, in fact! The real issue is that KSysLog currently has to run as root: apps run as root don't inherit user themes. The correct solution is to make it not need to run as root, and then make it request elevated privileges using PolicyKit only when necessary. You might talk to @stikonas, who recently did this work for KDE Partition Manager.
That video confirms what he mentioned - but it does provide a notion for a potential fix.
It seems like XDG_CURRENT_ENVIRONMENT is _not_ being set by or being used by kdesu and the launched application.
This can be workaround from the app itself. Current master branch of partition manager (which is not yet using Polkit) restarts itself with some environmental variables that are passed through.
Ugly but it served KDE Partition Manager for a while:
Whether kdesu keeps environmental variables actually depends on whether it calls su or sudo (kdesu can do both).