Thanks for calling this out. This is not something I would have thought to check myself. It's unfortunate the most common use of the word toxic is in a very abstract sense, but the behavior you identify is just plain unnecessary rudeness and a much more suitable use of the word.
Can we assume people are acting in good faith until we have reason to think otherwise? As highlighted by the green-is-yellow issue, it is OK to simply say, "Understood. The color scheme has been previously discussed, is popular and isn't currently considered for change. But you can configure it yourself by <link-to-documentation>..." The accusation of an entitled attitude appears a bit of a leap to me too. To be fair, while a tad rude in tone, the dev's second post is an improvement.
Or perhaps, "We don't have a lot of resources for this level of detail. Sorry." (Open-source honesty?) Or even, "If you wrote a patch taking the values from rxvt it would be considered but possibly rejected after discussing policy with lead devs."