A dimmed theme, with a more subdued UI with a little less contrast than our dark mode theme, is now available to all GitHub.com users as a public beta. This update also includes new appearance settings to give you more control over which theme displays during the day and night. 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" tab in your profile settings to choose your preferred theme setting.
