Just an FYI: On Linux, if you do the asset installation procedure, but installing to a custom subfolder instead of the tf/ folder, it will work fine, but custom icons will display as the black and purple checkerboard. To fix this, go to your custom subfolder, then go to "materials" and create a folder called "vgui". It does not need to have anything on it.
It'd be great if the Potato team could add this empty vgui folder to the asset pack.