These icons are based on the tutorial glossy orb web icons by devvv at www.gimpusers.de .
So I started to play around with this and you can see and if you like use the results below for free according to the Creative Commons conditions stated at the end of each box separately.
Plus minus icons
icons by Georg Schweflinghaus are licensed under a Creative Commons Attribution-NonCommercial 3.0 Germany License.
Blue orb icons
Book and document icons
icons by Georg Schweflinghaus are licensed under a Creative Commons Attribution 3.0 License.