I usually ignore the css files and just version the sass files and compile sass on deploys.
Here is a useful .gitignore file for backdrop:
# Ignore configuration files that may contain sensitive information. settings.php sites/*/settings*.php # Ignore paths that contain user-generated content. files private sites/*/files sites/*/private # Ignore sass stuff. .sass-cache stylesheets # Ignore PHPStorm stuff. .idea *.idea/*