The latest version of Firefox, version 63.0, features an automatic dark mode feature that switches Firefox to the dark mode automatically if you have enabled Dark Mode in Windows 10 File Explorer which requires the version 1809 of Windows 10. What if you are not using this version of Windows or if you want pass on the dark theme in Firefox for all the websites?
Dark Reader add-on for Mozilla Firefox makes this possible. This add-on can switch Firefox to dark mode even if you are not using the dark mode in Windows 10 and it enables the dark theme for all the websites. It is a great way to care for your eyes, especially if you are reading text content on the internet at the night time or in the dark surroundings.
After the installation of the Dark Reader add-on in Mozilla Firefox it puts an icon in the Firefox toolbar and enables the dark theme for all the websites. You can click on this icon to change the dark theme settings such as toggle between the dark or light theme, toggle brightness, contrast, sepia, grayscale, change font and so on. From this icon, you can also turn off the Dark Reader extension altogether. It also offers a hotkey Alt+Shift+D to toggle the extension off or on.
If you want to whitelist websites to always use the dark theme then you can add them in the site list. Similarly, you can add websites to never use the dark theme if the dark theme makes them look strange or harder to use. If you have installed any themes in Firefox browser, then Dark Reader allows you to switch to any of these themes from the convenience of its toolbar icon settings. You can switch to any of the installed themes or switch back to the default Firefox themes.
You can download Dark Reader add-on for Firefox from https://addons.mozilla.org/en-US/firefox/addon/darkreader/.