Fixed darken-by() BIF
Fixed darken-by() BIF

SimenB/stylus,meteor/stylus,peterkinalex/stylus-1,baotingfang/stylus,a-x-/stylus,PeterWangPo/stylus,Ricardh522/stylus,stylus/stylus,jameswomack/stylus,cemoulto/stylus,darwin/stylus,trixter5/stylus,jangarcia/stylus,yetone/stylus,Delapouite/stylus,Panya/stylus,angryrobot/stylus,johnnycopperstone/stylus,silvaemota/stylus,ThiagoGarciaAlves/stylus,Zagorakiss/stylus,modulexcite/stylus,tyv/stylus,HossainKhademian/Stylus,galenvagen/stylus,isaacs/stylus,rstacruz/stylus,behzad88/stylus,jonpacker/stylus