The craft writeup for this says to use a Sharpie, which would be easy enough, but you could also use sticker letters. For the Dark Side, line up your sticker letters as a mask, paint over them, then remove the letters.
What a fun addition to a man cave! Oooh, I should totally make one for my husband’s cave/office, then switch it out without telling him and see if he notices! :) The only part I don’t like about this is every time you have to leave the room, you have to “choose” the Dark Side!