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!-


Unfortunately that’s not possible with this tool (and not really recommended). Flatpaks are sandboxed and their bundles are read-only by design. Modifying them directly would break the sandbox and cause update problems. What kAppIcon can do is change the launcher icon that appears in your menu and panel for the Flatpak, that works fine. If you want to change the icon inside the running Flatpak app, you’d need a different approach (like runtime overrides or repackaging the Flatpak), which is outside the scope of this tool.
:-(