When you issue the grunt build command, Gruntfile.js processes various less files and outputs src/less/adapt.less. In turn, src/less/adapt.less is processed and becomes build/adapt/css/adapt.css. If you look through the tasks of Gruntfile.js, you may figure out ways to have a separate custom_style.less included during the build process. But until then, you may find it best to include your customizations in src/theme/adapt-contrib-vanilla/less/theme.less. You'll notice comments at the top that indicate this is the preferred place for bespoke/custom styles.