Flip the strips over onto some sort of a light table.
Outline the black lines with fine point marker, then using a wide tipped felt marker -
color in all the rest of the strip backing
outside of the symbol outlines....
It's tedious work but you'll love the results when you play your game.
After a day of drying, do them again if you're using a felt tipped marker
because you have to completely cover the black part.
If not, you'll see streaky light through the strips where it should be dark.
If you're good with oil-based paints and a smaller paint brush, then I suggest you try that
instead as it will only take one good coat of dark paint to completely opaque out the areas
you do NOT want light to shine through.
By the way, "opaque" means light will not go through.
I must strongly add that you do not want to get ANY paint on the symbol areas or
it will look even worse when the reel lights shine through.