In VS Code to see all files and/or elements a css class is being used,
search the workspace with the 'Search Panel'. For example, select the
class from css file, e.g. '.btn-primary', and goto 'Edit' on menu bar:
'Edit' > 'Find in Files' or 'Cmd + Shift + F'(Mac)
Use 'files to include' or 'files to exclude' fields to narrow search
e.g. search all html files and a 'script.js' within 'public/' folder:
'Search': .btn-primary
'files to include': public/**/*.html,script.js