To add custom styles, you need to modify a file called
Custom.css. You can find this file at the following locations on your computer:
%LOCALAPPDATA%\Google\Chrome\User Data\Default\User StyleSheets\Custom.css
/Users/YourUsername/Library/Application Support/Google/Chrome/Default/User StyleSheets/Custom.css
For the above style, check out this file from Darcy. He has documented almost all necessary selectors in that, so you can easily modify it and create your own styles.
Some more styles:
You can find more selectors at this URL: chrome-devtools://devtools/devTools.css. Just copy-paste it into the address bar of Chrome.