First things first, install User JS and CSS browser extension to use userscripts and userstyles.
You can use any userscript/userstyle extension (Tampermonkey, Stylish, etc.) but the one linked has the advantage to allow both JS and CSS/SCSS customization in a single extension.
for detailled explanation, check the official documentation of the extension
To install one the the script provided, follow the steps from the image below:
