Super Dark Mode for Chrome and Firefox is a free extension that allows you to apply a dark theme to websites.
It can be useful in helping you conserve battery life and eyestrain during long browsing sessions. Some overlays do not render correctly, making certain webpage icons/images incomplete or obscured. Still, the main body of the page does not appear to be affected.
Super Dark Mode for Chrome and Firefox doesn't take much to set up and does include some presets (filters) that you can use to customize it to specific sites and pages like Youtube, Facebook, Twitter, etc. Of course, you can turn it on/off for all if you prefer right from the top rich extension icon. It also permits whitelisting to keep them separate from any changes you make, displaying them in their default state.
IE Tab for Chrome is a free extension that effectively emulates the Internet Explorer rendering engine making it possible for you to visit IE required sites without actually launching IE.
Dark Reader is a free extension for Chrome, Firefox, Edge, Safari, and Thunderbird that will make surfing the web easier on the eyes through customizable dark themes.
The Super Simple Highlighter for Chrome extension will allow you to highlight selected blocks of text on a webpage, and restore them on each future visit.