i think this is a bug in power-manager and not the icon-theme or the panel, but the trayicon of power-manager gets scaled (image attached, above: screenshot, below: how it should be). the icon-theme i'm using it with is elementaryXubuntu-dark (https://github.com/shimmerproject/Bluebird/tree/master/icons), it has been reproducible for a few releases now and if i remember correctly was also present in xfce 4.6. is there anything i can do to debug this more?
k, so i debugged this a bit more. lxpanel exhibits the same problem, so i'd say it's most likely xfpm doing something wrong. after some more testing i found out that the trayicon is correctly displayed when it's in png format and scales when it's svg. guess this should be fixed in xfpm rather than adding a workaround in svg-based icon themes.
Nice, closing my own bugreport from a few years ago :) Anyway, trayicons -> panel-plugin.