I believe there are no plans to dump the light theme on GitHub, the dark theme was just an addition that many have been asking for.
Unfortunately many online services and software tools are oblivious to the accessibility needs of the many categories of people who have special needs (from those suffering from dyslexia, to the color blind, the blind, and many other forms of visual impairments).
I have reasons to believe that GitHub takes into consideration those needs.
In any case, there should be some browser extensions that are designed to re-adapt colors according to special needs (e.g. to compensate for different types of color blindnesses), so you might look for such an extension, which could ultimately help you enjoy any website, regardless of whether its authors adhere to the web consortium standards for web accessibility or not. Most of these tools rely on color profiles to adjust the colors visible in the browser to the special needs of various vision impairments, but this only works on browsers which do support color profiles (not all do, so you’ll need to do some tests there).