This is some CSS for exhentai.org.
It can be applied using the addon stylish you can download it for firefox here: https://addons.mozilla.org/en-US/firefox/addon/stylish/?src=hp-dl-featured or if you're on chrome or a chromium based browser you should use stylus, you can download it here: https://chrome.google.com/webstore/detail/stylus/clngdbkpkpeebahjckkjfobafhncgmne.
If you are on firefox just create a new style and copy and paste the code, save and name the style and you should be good to go.
If you are on chrome or a chromium based browser, you must uncomment the last few lines then create a new style in stylus, on the left you should see an option to import a style in the mozilla format, select that and paste the code into the box that appears.
You must install the font roboto: https://www.fontsquirrel.com/fonts/roboto or replace roboto with a font of your choice.
Colours can be easily changed at the top of the document to match your scheme.