I came back to KDE after a very long absence, drawn by my desire to return to an environment that could pretend I controlled. I remember tweaking all sorts of aspects of the (was it Enlightenment?) desktop back in the day. Want thicker window edges? Point and click.

Now, I can't find any way to customize anything about window edges, etc, and I REALLY WANT TO. Sure, I can download and try some other theme, but I can't really modify that either. I've found one or two themes that get me 70% there. Do I need to write code to modify these? I thought I must be getting simple minded. It is possible.

I've searched high and low for a KDE theme editor, and the last time that was discussed was in 2009. https://forum.kde.org/viewtopic.php?t=83569.

I understand that a lot of the previous KDE environments' customizability became increasingly difficult to support moving forward. Is that it? It seems now that defining a KDE theme is not a trivial undertaking. Is this correct?

I may have similar problems too. I am trying to reduce the padding between icons in the toolbars and there does not seem to be a user modifiable way. It has to be whatever the theme author set?

I am not a regular user of KDE these days so I am not very sure if it is actually a real issue or my being an idiot.

To modify/make a Plasma theme, you need a theme manager, I suggest you look for Kvantum Manager and/or QtCurve