Now that dark theme is available in Firefox, you can enable it at night to reduce the brightness of the screen that could cause strain on your eyes. But dark theme does not prevent a website to use white background or other light color themes. If you access a website with white background, then dark theme of Firefox does not help much in reducing stress on your eyes. If you want to apply the dark theme to all the websites loaded in Firefox too then you can use the Midnight Lizard extension for Firefox.
Midnight Lizard is a Firefox extension that allows you to change the color scheme of any website that you access in Firefox browser. It can also make the images embedded inside a website have different color saturation and brightness. Various elements inside a website like buttons and textboxes also are themed using a color scheme selected by the user.
After the installation of Midnight Lizard extension, you can click on its toolbar icon and enable it from there. It will immediately take effect and apply a dark theme to all the websites. You can change the color themes to be used for all websites or a specific site from its settings. You can download more color themes from their website. Other things you can customize include the background images, buttons, text, links, borders, images, fond images, scrollbars and more.
All of this is achieved through the modification of the CSS (cascaded style sheets) for any website in real time. Since Midnight Lizard has to monitor a website for changes continuously, it might make your Firefox web browser a little slow at times. Even though it makes websites appear really good, it can also break them sometimes, for example, DuckDuckGo’s logo disappeared when we tested Midnight Lizard on it.
You can get Midnight Lizard extension for Firefox from https://addons.mozilla.org/en-US/firefox/addon/midnight-lizard-quantum/.