One of the most requested feature in Divi theme is the contact form builder like contact form 7 plugin. The Divi contact form module is great but its not easy to customize and you are stuck with the form fields it has by default. Until a form Builder is added to Divi, the easiest solution is to use contact form 7 plugin.
The default look of a contact form built using the contact form 7 plugin is not very good. To make a form built using contact form 7 to look like Divi contact form module, copy the following code, and paste it in the custom css box in Divi theme options (epanel).