Hi,
if you change svg files in your zammad directory, then you will need to recompile your icon set. There is a compiled version with all svgs in public/assets/images/icons.svg.
I usually use this command to regenerate the icons:
zammad> cd /opt/zammad
zammad> cd public/assets/images && npm install && npx gulp build
In general it might also makes sense to have a development instance to include the generated files in a package (Packages Tutorial) and update it every minor version because else new icons will not be included.