Midnight Lizard

Midnight Lizard color schemes work across all websites. The extension offers many predefined color schemes to choose from, or you’re free to explore and install more at midnight-lizard.org
 ✔ Choose between dark, light, grayscale or colorful color schemes
 ✔ Modify existing or create new color schemes
 ✔ Use different color schemes on each website
 ✔ Adjust brightness, saturation, contrast and hues
 ✔ Improve accessibility and readability
 ✔ Increase contrast to make text easier to read
 ✔ Apply blue light filter to shift all colors towards red light
 ✔ Keep original images and hues without inversion
 ✔ Schedule automatic activation and deactivation
 ✔ Use keyboard shortcuts to toggle extension
  ⮚ on current website: Alt+Shift+L
  ⮚ globally: Alt+Shift+M
 ✔ Use with local HTML files
 ✔ Shade bright colors and images
 ✔ Change any colour you want
 ✔ Dynamically change browser theme (disabled by default)
 ✔ Sync your settings
 ✔ Configure background, text, buttons, links, borders and images
 ✔ Apply globally, per-website or with url match patterns
 ✔ Switch between websites blacklist and whitelist modes
 ✔ Use Simplified mode to improve performance on heavy websites
 ✔ Contribute to our open source project on GitHub.com/Midnight-Lizard
 ✔ Donate to our OpenCollective.com/Midnight-Lizard

Please use additional extensions in order to replace default New Tab page - extensions cannot style it. Also extensions don't work on New Tab page, Firefox Add-ons store and all Firefox system pages due to security concerns.

Please take a look at this tutorial: HOW TO improve Midnight Lizard performance