I thought this was supposed to be part of Plasma 5.13, that icons in Discover would follow the system icon theme. But it's not working for me on Arch Linux.
Which application? Are you sure the icon name is offered by appstream?
It does, but I see that Arch Linux has changed things a little. It looks like they prefix the package name to the icon name, like this: <icon type="cached" width="128" height="128">blender_blender.png</icon> I'm not sure Arch people will listen to me if I ask them to change their icon naming conventions...
No, that's not what we're after: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#sect-Metadata-GenericComponent We need to have the "stock" name available to display it on Discover. A bit of research though, suggests that blender doesn't provide any appstream information at all. https://www.archlinux.org/packages/community/x86_64/blender/ I guess it's time to request it to Blender.
Well, Blender _does_ have an appstream file, but on ArchLinux at least all the appstream info is in the archlinux-appstream-data package. In any case, very few applications in that database specify a "stock" icon. Even Kate's appstream data doesn't specify any icon---it appears that Arch developers had to add it to the appstream database. (https://cgit.kde.org/kate.git/tree/kate/data/org.kde.kate.appdata.xml) The same problem also appears to affect K3b, Gwenview, KTorrent, and probably others as well. Perhaps this problem could be brought to other (KDE) developers' attention somehow.