Hi,

Please try add more this code to Customize > Custom CSS to do that:

.post-entry .wp-block-quote,
.wpb_text_column .wp-block-quote,
.woocommerce .page-description .wp-block-quote {
   padding-left: 1em;
   border-left: 4px solid #000;
   background: #f7b4b4;
   color: #ffffff;
}
.post-entry blockquote.wp-block-quote p, 
.wpb_text_column blockquote.wp-block-quote p{
    font-size: 18px;
}

Change the color codes  f7b4b4 and ffffff to background & text color for blockquote. You can get a color code here

Change value 18 to font size as you want.

Best Regards,

PenciDesign