#1. First, you need to find the Simple List Section ID, use this free tool
In my example, you will have:
#2. Suppose you want to target Item 2, it should be
section[data-section-id="66058810a2d50f0d71e77dcf"] li:nth-child(2)
#3. Some example
To hide item 2, use the Custom CSS code like this
/* Hide item 2 */ section[data-section-id="66058810a2d50f0d71e77dcf"] li:nth-child(2) { display: none; }
To change the background color, use Custom CSS code like this
/* change item 2 background */ section[data-section-id="66058810a2d50f0d71e77dcf"] li:nth-child(2) { background-color: #f1f; }