Change product excerpt font size - 7.0 Mentor theme

Site URL: https://www.genderandradiation.org/products

This is an older site, using the Mentor theme in 7.0.
Can anyone tell me how/where to change the font size of a product excerpt/description?  In Site Styles, the only option appears to be either Show or Not Show the Excerpt, no styling options. Any guidance much appreciated!

Add to Design > Custom CSS

/* product excerpt font */
.ProductItem-details-excerpt p {
    font-size: 12px;
}