This is indeed not yet documented anywhere.
What you need to do is to add your custom icons in conf/icons/classic. Note that the Classic UI uses by now SVG icons, so you would need a file conf/icons/classic/switch.svg.
If you just want to replace some icons by others, you can find the full existing icon set at https://github.com/openhab/openhab2/tree/master/bundles/ui/org.openhab.ui.iconset.classic/icons.
2 Likes