Added green theme to theme switcher

Based on color psychology by Nick Kolender (http://www.nickkolenda.com/color-psychology/), I have chosen a simple paleish green as backgroundcolor, because green is a relaxing color and a paler color is easier for the eyes. The intended use of ARSnova.click is to have a fun and easy to use tool to create a quiz,that is why I think green is a good color for the webapp. The orange buttons draws attention to them to make it more easy for the user to guide through the webapp. I choose the standard green for correct and red for wrong due to it being considered normal and every user is used to it by daily life. The blue button for "general" settings like formatting is to have a contrast between necessary steps like creating a quiz and more optional steps like formatting your text