CSS changes are visible while editing but disappear on refresh

I'm trying to add a 1px border to some of the images and the changes would be visible while I'm editing but, as soon as I refresh or navigate to another page, they'd not be visible anymore. For example on the attached image of this Homepage component. 

The code I'm writing is looking like this:

div#yui_3_17_2_1_1579085870035_321
{
border-left: 1px solid black;
}

Is that something I'm doing wrong?

My site (running on 7.1) : https://pelican-chiton-xwy4.squarespace.com/