I made a small tool for KDE Plasma (and other freedesktop desktops) that lets you easily customize single app icons:

• Use your own images or library
• Copy icon from another app
• Pick one icon from any installed theme (without switching global theme)
• Built-in pixel editor to create your own
• Batch apply, undo, overrides, missing icons overview
• Import image to turn it into an icon

Completely user-level, safe, and reversible. No manual .desktop editing.

GitHub: https://github.com/rayman1972/kappicon

AUR: kappicon or kappicon-git

Feedback welcome!-