Source: http://www.elegantthemes.com/blog/tips-tricks/9-useful-divi-css-snippets-you-can-add-to-epanel-in-seconds

.et_pb_toggle_open .et_pb_toggle_title:before {
transform: rotate(180deg);
content: "\25bc";
transition:all 0.5s ease-in-out 0s;
color:#e5e2d3;
}
.et_pb_toggle_title:before {
content: "\25bc";
font-size:2em;
transition:all 0.5s ease-in-out 0s;
color:#179baf;
}Code language: CSS (css)