Hello. I have just installed Layan theme but window corners has some sharpness. I have also tried other themes and then I have realized that the problem in window decorations. Only Breeze and other preinstalled window decorations work normally.
It's hardly visible in screenshot.
https://ibb.co/T1twKmB

Can I fix this?

KDE Plasma Version: 5.23.4
KDE Framework Version: 5.89.0
Qt Version: 5.15.2
Kernel Version: 5.15.12--MANJARO
X11

Nope. This is a long lasting issue. Corner overlay. Layan is a aurorae theme while Breeze is more a window application. However, there have been attempts. Like this one ( haven't tried it yet cause I don't do aurorae themes). Might give it a try. https://github.com/Alban-Boissard/kwin-effects-blur-respect-rounded-decorations

Thank You for answer. I have decided to stay with Breeze for a while. May be In future I will change theme if there won't be an issue like this

New LightlyShaders on the github. This worked for me, maybe it will work for you.

Plasma >= 5.23.

https://bugs.kde.org/show_bug.cgi?id=395725

This issue has been fixed in plasma 5.25, It works for me with Sweet Mars KDE theme on Arch Linux