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.