This is not an A/B test - those happened a while ago and it has now been rolled out to all sub's. There has been a bunch of stink raised over it on https://meta.stackexchange.com/, but then that seems to be a place where a lot of stink is raised frequently.
It seems like there are some overall colors that make up a palette for each sub, and the arrow coloring is drawn from one of those. Math.se seems to be one for which the lack of contrast is significantly worse. I've posted in those meta.se questions about our particular issues.
(What follows is speculation.) It looks like they put a lot of time and energy into designing and testing this change. With that much sunk cost it seems unlikely that they'd "oops, sorry, rollback" the change. The only response I've seen from company people is "Thank you for bringing this to our attention. Our team is working on updating the colors of the button after voting on Meta sites so it will be much more visible." I've also seen somewhere where "near future" was mentioned as the timeline, though I can't seem to find that now. Use that as you will when deciding if you want to try to come up with your own work-around, or wait to see if/how it is "fixed".