Yeh, I ended up sniffing through that code. At least it told me what get_icon() defaults with its arguments.
Right now I am bread boarding another plugin element (in gdscript) but running it rather than trying it out from the editor (so I can debug). For this I saved the editor theme and assigned it to the component I am designing.
I poked around more in the docs and found that when operating as a plugin, you can use the inspector interface to get at the theme.
thanks for the reply!