if i test my page on : https://securityheaders.com/?q=https%3A%2F%2Fwebers-webdesign.de&followRedirects=on

you can read the description of:

x-content-type-options: X-Content-Type-Options stops a browser from trying to MIME-sniff the content type and forces it to stick with the declared content-type. The only valid value for this header is “X-Content-Type-Options: nosniff”.

So your advice is to change the mime types for those generated post css to : text/css

but as far i can see on : class-post-css-management.php – line 919:
return '<style type="text/css">' . $this->new_ln . $rules . '</style>' . $this->new_ln;

isn’t this responsible for those css settings?

__________

i set the filter ( avf_post_css_create_file ) now on one of my testinstallations to true ( only then files are generated inside dynamic-avia folder) – and switched of merging.
On dev tools i checked if i find some text/html in DOM – but nothing to find.

and your layerslider got the id 8? and nomenklatura is there a bit different #layer_slider_X
on default the first on a page will have : #layer_slider_1
the next #layer_slider_2

so you can have:

@media only screen and (min-width: 768px) {
  #layer_slider_1{
    display: none!important;
  }
}

@media only screen and (max-width: 767px) {
  #layer_slider_2{
    display: none!important;
  }
}

check your page with developer tools which ID those sliders have.

_______

or you can have both videos in one layerslider and choose there the visibility.
you have on each layer on the right side the tabs of layersettings – and on content tab you can set the layervisibility for desktop, tablett, mobile “toggle device visibility”

oh sorry – then I misunderstood your request. To me it sounds like you want to see the icons inside the mobile menu as well (hamburger menu). – The above way is a quick and easy way to have this for small social icon lists. I didn’t see that these are hidden below. of 479px.

__________

or try to preserve the social-bookmarks on the same place – because even on 280px screenwidth ( most mobile phones got 320px and more ) the logo does not overlap that area:

@media only screen and (max-width: 989px) {
  .responsive.av-burger-overlay-active #top #header .main_menu .social_bookmarks a {
    color: #FFF;
  }
/** if you had changed the phone-info input field - see at bottom of my message **/
  .phone-info .header-meta-mail {
    display: none;
  }
}

@media only screen and (max-width: 479px) {
  .responsive #top #wrap_all .main_menu .social_bookmarks {
    display: block !important;
  }
  .responsive #top #wrap_all .main_menu .avia-menu .av-burger-menu-main {
    border-color: #1b5289;
    padding-right: 15px;
    margin-right: 5px;
    border-right-width: 1px;
    border-right-style: solid;
    transition: border-color 0.2s ease-in-out;
  }
}

but i guess you have to change then a bit your phone-info input field. The standard behavior of enfold between 768px and 989px here in your setting is to show the burger icon and the social-bookmarks besides each other and to hide header_meta social bookmarks, so the “of mail” is obsolete on top then.
maybe you can do this:
<div><a href="tel: +31595729729">Bel 0595 729 729</a><a class="header-meta-mail" href="https://noorderwindbewindvoering.nl/contact/#contactformulier"> of mail</a></div>
– and with that class we can hide it on demand

or in my opinion better – just to add the binding word to e-mail bookmark without link:
<div><a href="tel: +31595729729">Bel 0595 729 729</a><span class="header-meta-mail"> of mail</span></div>

i could not test it now if it works – but the code from : https://kriesi.at/support/topic/accordion-slider-stop-allowing-all-images-to-show-in-slider/#post-1435166 does not lead to a functions.php error on my end here – means – the syntax is o.k.

yes – where is the leading:
function ava_custom_script_group_lightbox()

on : link

put this to your quick css:

@media only screen and (max-width: 989px) {
  .responsive.av-burger-overlay-active #top #header .main_menu .social_bookmarks a {
    color: #FFF;
  }
}

Had same issue, check here: https://kriesi.at/support/topic/blog-add-header-transparent-to-archive-page/