Header Transparent on Mobile and Tablet devices

Dear Support Team
For the website: http://neu2024.schollmeyer.dk the header in the mobile and tablet version (as well as in the desktop version) should be transparent at the beginning (white logo and the burger menu should then also appear in white).

I have inserted the following script in the css:
/* Transparent Header For Mobile */
@media only screen and (max-width: 990px) {
.responsive.html_mobile_menu_tablet #top #wrap_all #header {
position: absolute;
}
.html_mobile_menu_tablet #top #wrap_all .av_header_transparency {
background: transparent;
}
}

But unfortunately it doesn’t work > see screenshot: http://neu2024.schollmeyer.dk/support/LEISTUNGEN.jpeg. How do I have to change it so that the header also appears transparent in the mobile and tablet version when the website is called up?

Best regards, Diana