Both parent and child CSS are enqued through functions.php. The plugin minifies the child CSS twice, and does not minify the parent CSS.
(mbsimplicity theme is child of modularbase theme - parent CSS called by @import in child CSS)
/*! This minified dependency bundle includes: * 01. http://demo.modularbase.com/wp-content/themes/mbsimplicity/style.css * 02. http://demo.modularbase.com/wp-content/themes/mbsimplicity/style.css */
When performing tests on page tests for "Website Speed and Performance Optimization" on sites such as GTmetrix (gtmetrix(dot)com), it shows the parent CSS not being minified.