How to change text box background overlay of default Home?

Please use the following CSS code snippet in the Divi Theme Options Custom CSS area:
(Replace the last value from 0.5 to a lower number for higher visibility.

For example: 0.3 or 0.2)

@media(max-width: 767px) {
.dg-main-header .header-content:before {
background-color: rgba(255, 255, 255, 0.5);
}
}
.dg-main-header .header-content:before {
background-color: rgba(255, 255, 255, 0.5);
}

This is the output with value rgba(255, 255, 255, 0.2)