Magento makes use of CSS preprocessing using LESS. There are two modes of Magento 2 LESS Compilation: client side and server side. Both modes involve compiling .less files into .css files.
To learn more about writing LESS, view the Getting Started with LESS post.
Magento’s blank theme includes three main stylesheets that are included via …
One of the most common tasks a frontend developer will look to undertake is how to add custom CSS in Magento 2.
Like earlier versions of Magento, CSS files are added via layout XML. Within Magento 1, in your custom theme you could specify a local.xml layout file that contained all of layout updates.