Work with the information you have: you know the color is turning black so you can search for the rgb or hex value that corresponds to dark colors.
Hexadecimal order is like that: 0123456789ABCDEF, 0 is the first number F the last
000000= Black
ffffff= white
So youre searching for a number that mostly uses the values 0123 at the beginning.
This could look like that: 030303 or 101010 or 2e2e2e.
RGB uses numbers up to 255
0,0,0= black
255,255,255= white
So youre using for numbers that mostly use values that are 1 or 2 symbols long.
Now you have to search in the snippet itself and change this. The good thing of css. snippets is that obsidian is updated as soon as you save the changed snippet. You dont need to close or update the obsidian window neither do you have to close the snippet file to see changes. In your case you only have to hoover over an item.