I have a owncloud installation since 8.x.
In every update I’ve make the theming with the line
and making a themes/name_theme folder.
Now with 10.0.3 i find a apps/theme-example where are a copy of /core and /appsinfo.
I think this was a ‘example’ to copy to the themes/ but I’ve find that now the .css and images must be there (at apps/theme-example) to work the theme.