A colleague is strongly opposing, and will not let me merge it until I find a customer that contacted the company because it's affected by this issue.
What can I do?
More seriously, working with people who don't care about what they're building enough to want it to be a better product is hard. I don't really want to recommend that you resign and find a new job, but two decades of experience writing web software tells me that's probably the only real solution. If you're on a team with people who don't actively want the code to be as good as possible, and who are willing to put the effort in to making it that way, will be bad for your mental health and possibly bad for your career if you pick up that habit.
Tossing aside the accessibility angle, fly by night changes often have ramifications that the authors did not anticipate. I have and would again veto change requests that not part of our planning process and offered as: "Surprise, here is this cool thing I did."