A dark high contrast theme, with greater contrast between foreground and background elements, is now available to all github.com users as a public beta. Opt-in to the beta by enabling the theme in your Feature Preview settings, located in the profile menu dropdown. Once the Feature Preview has been enabled, navigate to the "Appearance" page in your profile settings to choose the dark high contrast theme.
Please note, you may notice color issues with other themes when in this beta. If they are disruptive you can opt-out by disabling in Feature Preview.