That kinda is the whole point of static releases like debian stable, ubuntu lts, red hat or windows ltsc? That you actively don't want to have any changes like these but want stable (=static) foundations to build on. (currently writing this on old stable; don't feel bothered to upgrade right now)
> I think Plasma has something like "Decorations" - can a custom decoration be used? Can one use scripts in the "Decorations"?
The software under discussion seems to be a change to the breeze window decoration so for this one probably not (yet), but e.g. for aurorae
compiz is famous for including all sorts of fancy/over the top effects like windows going up in flames, so stuff like glowing window borders should be no problem at all.
By now compiz seems mostly unused, but many effects are also available in kwin.
For instance
system settings -> desktop behavior -> desktop effects -> dim inactive
Kinda the opposite of outlines, where it just dims everything else instead of highlighting one, but maybe it works for you?
Because it does something different? The new one changes colors depending on the background color, for a different window decoration, outline instead if borders (rounded corners etc.)...
How does one use it?
Is it a script I can put somewhere on my laptop and then the borders will have outlines?