Firefox for Android does not have a convenient interface for modifying colors. I think it can be done using about:config but I wonder whether you might want to try an add-on for this.

menu > Tools > Add-ons > Browse all Firefox Add-ons

Then search for color and check out "Remove Document Colors" to see whether that does what you need. (I haven't tried it myself.)

Hi

I recomend trying this Add-on.

I hope this helps, but if not, please come back here and we can look into a different solution for you.