frontend.min.css file is loaded by Squirrly SEO to define the style for the Twitter and Article box used to insert tweets and blog quotes in your articles.
The Twitter and Article boxes can be inserted from the Squirrly Copyright Tool.
They look like this after you insert them in your post:
If you don’t use them and you want to prevent the CSS from loading, simply add in wp_config.php:
If you use a cache plugin, purge the cache after you save the wp_config.php file.