In developer mode I been getting the error
FileError: '/pub/static/frontend/theme/theme_name/en_GB/css/styles-m.css' wasn't found (404)
I've taken the following steps to try and resolve this:
Used Refresh static content in the cache management
Taken magento2 out of development mode, check if worked then back in development mode
Backup current static folder, removed contents inside and try to refresh.
All of these points still give me the error above
php bin/magento setup:static-content:deploy
command from command line it may resolve your problem