How to create costumes without alpha with pen

yea weird title but thats what I need.
Lets say I drew a picture on my black background with transparent pens. I wanna make it my background's costume. but when I do that with
Hydraulic Erosion test script pic
it makes it on white board. but I want background to be black. because i am drawing with mostly whites.

is there a way to change default background color or idk like sticking two costumes together (pen drawings to black background costume)

okay okay. I hate myself because I always find the solution right after I post it on somewhere. all I needed to was first filling the screen with black via pen. so its like also saving the background

Or stamp a big black (or another color) sprite on the screen before...

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