Any reason why custom CSS is not inherited on my live site?

Site URL: https://www.steminnovations.co/about

Just wondered if anyone else had an issue where their custom-css was not visible on the live site, but is visible in preview mode? 

I added this, so that the text would sit beside the image on mobile view:

@media screen and (max-width:767px) {
#yui_3_17_2_1_1602503847421_274 {
    float: left !important;
    width: 50% !important;
  }
  
#yui_3_17_2_1_1602503847421_274.row>.col:nth-child(2n+1) {
    clear: left!important;
}
}

The preview showed exactly what i was looking for, but the live site on mobile didn't have any effect. 

#yui- is dynamic ID. It will change if you reload the page

Use #block-yui

Use this extension to find block yui id. https://chrome.google.com/webstore/detail/squarespace-collectionblo/bggpdfnccodbnmcndckmeehdjkjojkde

oh neat! Thanks @tuanphan