View Websites in Dark Mode
The first thing you'll need in order to view websites in dark mode on Safari is Cascadea. It is a macOS app which dynamically re-renders websites without a built-in dark mode using custom CSS rules. You can discover community-curated custom CSS rules from various sources including GitHub & Stylish.
GitHub
You can view GitHub in Dark Mode using GitHub-Dark. You can even adapt it to work with a GitHub Enterprise distribution if you change the regex in the rules from "github.com" to "github.enterprise.com". Take caution, though, to use an appropriate version of GitHub-Dark for your GitHub Enterprise version. For instance, the last known good version of GitHub-Dark for GitHub Enterprise 2.20.11 is 2.1.3. If you're not sure which exact version to use, you can binary search your way through GitHub-Dark releases until you find one that looks mostly right.
You can customize the style further with variables:
- GitHub syntax theme: One Dark
- CodeMirror syntax theme: One Dark
- Background: None
- Code Tab size: 2
- Code Font: ui-monospace (to use the system monospace font)
- Code Wrap: Wrap