Why is my top-header on top of my navigation menu? - css

Question

I have created a custom top-header with CSS ID 'top-header'. But, it is showing up on top of my navigation menu instead of below it.
I have tried changing the top-header position from 'absolute' to 'fixed', as well as change the top pixels from 3px to 300px but nothing changed.
Edit: I am using Divi to build a website.
I input the following code to the bottom of the functions.php:
function custom_header ( $main_header ) {
$custom_header .= do_shortcode('[et_pb_section global_module="38561"][/et_pb_section]');
return $custom_header . $main_header;
}
add_filter( 'et_html_main_header', 'custom_header' );
Any suggestions?
Photo of top-header on top of navigation menu
#main-header {
box-shadow: 0 0px 50px rgba(0, 0, 0, .1);
}
#main-header.et-fixed-header {
box-shadow: 0 0px 25px rgba(0, 0, 0, .1) !important;
}
#top-header #et-info {
position: absolute;
top: 3px;
}
#et-info-phone:before, #et-info-email:before {
margin-right: 8px;
position: relative;
top: 2px;
}
#et-secondary-menu {
position: relative;
top: 2px;
}
#et-secondary-menu a {
font-size: 18px;
}
#media screen and (max-width : 600px) {
#top-header #et-info {
line-height: 1.5em;
}
}
/*
/////////////////////////////////////////////////
NAVIGATION
/////////////////////////////////////////////////
*/
#main-header .nav li ul a {
color: #000;
}
#main-header .nav li ul a:hover {
background-color: transparent;
}
#top-menu .menu-item-has-children > a:first-child:after, #et-secondary-nav .menu-item-has-children > a:first-child:after {
top: -2px;
}
li.sfly_social_media_menu_icon .fab {
font-weight: 300;
}
#et_top_search {
margin-top: 0;
}
#top-menu ul.sub-menu {
width: 300px;
box-shadow: 0 20px 80px rgba(0,0,0,.2);
}
#top-menu ul.sub-menu li {
font-size: 12px;
line-height: 1.2em;
}
#top-menu ul.sub-menu li a {
width: 260px;
padding: 15px 20px;
}
#media screen and (max-width : 980px) {
.et_mobile_menu {
background-color: rgba(0,0,0,.9);
box-shadow: 0px 25px 50px 0px rgba(0, 0, 0, .25);
text-align: center;
}
.et_mobile_menu li a {
color: #fff !important;
text-transform: uppercase;
font-size: 24px;
font-weight: 700;
letter-spacing: 4px;
padding: 15px 5%;
}
.et_mobile_menu li li {
padding-left: 0;
}
.et_mobile_menu li li a {
font-size: 18px;
padding: 6px 5%;
}
.et_mobile_menu li li:last-child a {
padding-bottom: 12px;
}
.et_mobile_menu li.sfly_social_media_menu_icon, .et_mobile_menu li.sfly_social_media_menu_icon a {
display: inline-block;
}
.et_mobile_menu li.sfly_social_media_menu_icon a {
padding: 25px 10px;
}
}
#media screen and (max-width : 600px) {
.et_mobile_menu li.sfly_social_media_menu_icon a {
font-size: 20px;
}
}