Fix plus icon to not have black background
Fix plus icon to not have black background

Contributes to #325

ad1217/Cura,fieldOfView/Cura,totalretribution/Cura,markwal/Cura,ynotstartups/Wanhao,fxtentacle/Cura,fxtentacle/Cura,totalretribution/Cura,senttech/Cura,Curahelper/Cura,bq/Ultimaker-Cura,markwal/Cura,fieldOfView/Cura,bq/Ultimaker-Cura,hmflash/Cura,senttech/Cura,ynotstartups/Wanhao,hmflash/Cura,ad1217/Cura,Curahelper/Cura