Basically, you just need to add border=F to the hist function to remove the border of histogram bars.
# Create data
my_variable=c(rnorm(1000 , 0 , 2) , rnorm(1000 , 9 , 2))
# Draw the histogram with border=F
hist(my_variable , breaks=40 , col=rgb(0.2,0.8,0.5,0.5) , border=F , main="")
