To change the style of the post titles in the Divi Theme, you use CSS like the following:

.single h1 { 
    font-family: Courier;
    font-size: 30pt; 
    color: green; 
    border-bottom: 1px solid #ddd;
} 

This example change the post title font to Courier, makes the font size bigger, makes the title green, and adds a thin grey line under it (by adding a bottom border).