said Thierry de Coulon via tde-users:
| Are you sure it's .svn? I can't find a graphic format named svn.
Sorry, it is indeed .svg. but apparently not just any .svg. They are in /usr/local/share/pixmaps and, as an example I'll use protonmail. It's protonmail.png. But if I open it in the GIMP, I get a popup that says "Load SVG image" which I can do. The same phenomenon exists across the Proton apps. They work uneventfully as icons in other desktops, but in TDE they don't, producing either a white blob or the gear icon, even when installed by hand.