Icon helper not working with Zathura and Thunderbird

[paulhersch]

Hi,
I don't know if the icon_helper.lua from the helpers is something official, but i used it anyways.
Works fine except for Zathura and Thunderbird, where the imagebox which should display the images doesn't show anything. Icons are both reported via xprop and exist in my icon theme (Papirus) so i am kind of unsure why the icons are not being found.
Any ideas why this happens? I can share the code of the widget if that helps, its a little long tho.