How to modify CSS files in Magento 2

In Magento 2, do not modify CSS files within pub/* or vendor/* directories, because pub is for deployment and vendor is for default structure. Instead:

• Create a new theme in app/design/frontend/{vendor}/{yourTheme}/, as well as a new theme which inherits from Blank.
• Edit an appropriate .less file. Don’t get replaced when clearing the cache or upgrading the system.
• Utilize Grunt for compiling .less into deployment files.
• Setup sourcemaps to pinpoint your styling within the theme .less files. This step improves your productivity.

Source: stackexchange question, bb8d

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