There are PHP notices inside your CSS file, which makes your CSS invalid. Looks like your theme has some sort of CSS compiler but it’s not working properly. You should probably sort that out first with the theme author.
<br /><b>Notice</b>: Undefined index: webnus_container_width in <b>D:\xampp\htdocs\wordpress\wp-content\themes\mexin-wp\inc\dynamicfiles\dyncss.php</b> on line <b>13</b><br />
P.S. It’s hard to see anything when CSS is minified.