The GitHub Primer design system underlying GitHub UI frontend was recently updated to enable dark mode theme, which was no small task.

Shipping dark mode meant rebuilding and redesigning core foundations of our design system, and finding ways to refactor parts of GitHub that were never designed with themeability in mind. It took some of the best people I've ever had the pleasure of working with...

— Diana is broccolini (@broccolini) December 8, 2020

Previously we used web browser accessibility settings to achieve a similar effect, but the new dark theme is more consistent and effective. Some GitHub apps use icons that don’t have good contrast with the dark theme, but presumably the apps will update soon.