Deploy your static content on development mode for easy to develop your theme.

If your website is in developer mode, and you run bin/ magento dev:sourcetheme:
deploy, most files in this directory will be symlinked to their
sources throughout the Magento codebase. In production mode, you run
bin/ magento setup:static-content:deploy to process and copy files to
this directory.

Puneet Kumar Magento Developer