Source: https://divibooster.com/add-icons-to-accordion-module-titles/

Edit the toggle and go to Advanced » Custom CSS » Module elements » Before. Then add the following CSS:

background: #ffe600;
border-radius: 150px;
padding: 3px 10px;
color: #171717;
content: '✔';
float:left; 
font-size: 1em;
font-weight: 600;
margin-right: 0.5em;
Code language: PHP (php)