Adding new less imports in Magento 2 in Development mode

Importing a new less file when extending a Magento theme needs a few steps:

• Create the new file in your theme (in the app/design/frontend/<vendor>/<theme>/web/css/source/ folder)
• Add an import directive for that file in your normal _extend.less file with //@magento_import '_newfile.less'; (yes, with the // - see here)
• For development mode, build the files using the grunt exec command†. This creates symlinks in your pub/static/frontend/<vendor>/<theme>/<locale>/css/source/ folder
• For production mode, build the files using the bin/magento setup:source-theme:deploy command. This copies the less files into the pub/static/frontend/<vendor>/<theme>/<locale>/css/source/ folder

That should be it.

† - I’m pretty sure the bin/magento dev:source-theme:deploy command should cover this, but for some reason it didn’t work for me