I was trying to get this work as well. See my post
https://discourse.roots.io/t/css-files-not-compiling/14890
I ended up just manually copying the css files from the contact form 7 plugin folder and adding them into my partial scss files.
I updated the .ajax-loader line to:
background-image: url("/plugins/contact-form-7/images/ajax-loader.gif");
Obviously not ideal but I’m just going to do it this way for now until I get a chance to rebuild my forms without a plugin.