***UPDATE****
The new options that came with Divi 2.4 allow for greater control over the Divi header, including making it transparent. Check out the Divi header options here.

To make the Divi main menu transparent you can add the following CSS code into your ePanel or style.css file of your Divi child theme,

#main-header {

color:#000;
opacity: 0.8 !important;
}

0=fully transparent

1=solid background

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