I think you've posted the wrong link
Sorry, but I've changed the link now.
Love the way you've simulated the dark letters of a real physical display 
You have deviated a bit from the challenge restrictions by changing the global variable letters and you don't seem to have used a custom block to set a pixel to a colour.
I've made those restrictions a bit clearer in the original post
[edit 12Apr2024 - just went thru your code and I do appreciate the clever technique you've used ]
here's my attempt:
(i didn't use the block to set pixel color - sorry)
Doesn't seem to do much when I run it
sorry about that - let me work on it some more.
i fixed it (i think).. please try again.
Looks good now