Unfortunately, my EndeavourOS computer doesn’t support Bluetooth, so a Bluetooth icon doesn’t appear on my Xfce panel. In particular, I don’t know the name of the Bluetooth icon, so I can’t reference it in a GTK+/CSS selector.
I’m a bit overwhelmed with work and other obligations at the moment, but I did find the time today to write a tutorial that explains the approach I use when I want to resize, reposition or otherwise transform an Xfce panel icon: