The Contact Form 7 plugin is a popular form plugin for WordPress and Agent WP has written this helpful CSS to integrate Contact Form 7 with DIVI.

.wpcf7-text, .wpcf7-textarea, .wpcf7-captchar {
background-color: #eee !important;
border: none !important;
width: 100% !important;
-moz-border-radius: 0 !important;
-webkit-border-radius: 0 !important;
border-radius: 0 !important;
font-size: 14px;
color: #999 !important;
padding: 16px !important;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
box-sizing: border-box;
}
.wpcf7-submit {
color: #f0853b!important;
margin: 8px auto 0;
cursor: pointer;
font-size: 20px;
font-weight: 500;
-moz-border-radius: 3px;
-webkit-border-radius: 3px;
border-radius: 3px;
padding: 6px 20px;
line-height: 1.7em;
background: transparent;
border: 2px solid;
-webkit-font-smoothing: antialiased;
-moz-osx-font-smoothing: grayscale;
-moz-transition: all 0.2s;
-webkit-transition: all 0.2s;
transition: all 0.2s;
}
.wpcf7-submit:hover {
background-color: yellow;
border-color:yellow;
padding: 6px 20px !important;
}
</code>