or is it this you like to achive: https://webers-testseite.de/cynthia/video-under-header/
: a kind of watermark

insert to your slide an overlay : no color but an image – set transparency on it you like.
After that you can style it to be not as a pattern but as one image centered:
f.e:

 div .av-section-color-overlay {
    background-repeat: no-repeat !important;
    background-position: center center;
}

play with that background-position if you like –
first value is : background-position-x
second value is: background-position-y

i do not see your private content (i’m participant as you) but to solve that issue we had to see the site or have more details.
What kind of header you have ( logo left – menu right etc) ?
is it a fixed header or does it scroll away with content?
Does the content of that extra container have dynamic content or is it static and not often changed?
What should happen to that extra container on scroll?

Yes – but i would advice you to have “buttons” on small screens underneath each other.

if you know how to insert the shortcode here is the shortcode of that layout:

[av_layout_row border='' min_height_percent='percent' min_height_pc='35' min_height='0' color='main_color' mobile='av-flex-cells' id='' av_element_hidden_in_editor='0' mobile_breaking='' av_uid='av-jz6fxoy1' custom_class='']
[av_cell_one_third vertical_align='middle' padding='30px' padding_sync='true' background='bg_color' background_color='#38a8da' background_gradient_color1='' background_gradient_color2='' background_gradient_direction='vertical' src='' attachment='' attachment_size='' background_attachment='scroll' background_position='top left' background_repeat='no-repeat' link='manually,#' linktarget='' link_hover='' mobile_display='' av_uid='av-jz6fvtyh' custom_class='']

[av_font_icon icon='ue84d' font='entypo-fontello' style='border' caption='' link='manually,#' linktarget='' size='40px' position='center' animation='' color='#ffffff' av_uid='av-jz6fyjec' custom_class='' admin_preview_bg=''][/av_font_icon]

[av_heading heading='— Renting Process —' tag='h2' link_apply='' link='manually,http://' link_target='' alt_attr='' style='blockquote modern-quote modern-centered' size='' subheading_active='' subheading_size='15' id='' margin='' padding='10' color='custom-color-heading' custom_font='#ffffff' custom_class='' admin_preview_bg='' av-desktop-hide='' av-medium-hide='' av-small-hide='' av-mini-hide='' av-medium-font-size-title='' av-small-font-size-title='' av-mini-font-size-title='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' av_uid='av-1fveu9n'][/av_heading]

[/av_cell_one_third][av_cell_one_third vertical_align='middle' padding='30px' padding_sync='true' background='bg_color' background_color='#a7a7a7' background_gradient_color1='' background_gradient_color2='' background_gradient_direction='vertical' src='' attachment='' attachment_size='' background_attachment='scroll' background_position='top left' background_repeat='no-repeat' link='manually,#' linktarget='' link_hover='' mobile_display='' av_uid='av-jz6fvyc9' custom_class='']

[av_font_icon icon='ue80b' font='entypo-fontello' style='border' caption='' link='manually,#' linktarget='' size='40px' position='center' animation='' color='#ffffff' av_uid='av-jz6fyjec' custom_class='' admin_preview_bg=''][/av_font_icon]

[av_heading heading='— Sellers Guide —' tag='h2' link_apply='' link='manually,http://' link_target='' alt_attr='' style='blockquote modern-quote modern-centered' size='' subheading_active='' subheading_size='15' id='' margin='' padding='10' color='custom-color-heading' custom_font='#ffffff' custom_class='' admin_preview_bg='' av-desktop-hide='' av-medium-hide='' av-small-hide='' av-mini-hide='' av-medium-font-size-title='' av-small-font-size-title='' av-mini-font-size-title='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' av_uid='av-16oz9iz'][/av_heading]

[/av_cell_one_third][av_cell_one_third vertical_align='middle' padding='30px' padding_sync='true' background='bg_color' background_color='#e83947' background_gradient_color1='' background_gradient_color2='' background_gradient_direction='vertical' src='' attachment='' attachment_size='' background_attachment='scroll' background_position='top left' background_repeat='no-repeat' link='manually,#' linktarget='' link_hover='' mobile_display='' av_uid='av-jz6fw4u3' custom_class='']

[av_font_icon icon='ue821' font='entypo-fontello' style='border' caption='' link='manually,#' linktarget='' size='40px' position='center' animation='' color='#ffffff' av_uid='av-jz6fyjec' custom_class='' admin_preview_bg=''][/av_font_icon]

[av_heading heading='— Buyers Guide —' tag='h2' link_apply='' link='manually,http://' link_target='' alt_attr='' style='blockquote modern-quote modern-centered' size='' subheading_active='' subheading_size='15' id='' margin='' padding='10' color='custom-color-heading' custom_font='#ffffff' custom_class='' admin_preview_bg='' av-desktop-hide='' av-medium-hide='' av-small-hide='' av-mini-hide='' av-medium-font-size-title='' av-small-font-size-title='' av-mini-font-size-title='' av-medium-font-size='' av-small-font-size='' av-mini-font-size='' av_uid='av-kbfs1n'][/av_heading]

[/av_cell_one_third]
[/av_layout_row]

you can decide if you like to have column link or link on icon and heading ( you then must synchronize it)
looks this way: https://webers-testseite.de/pseudobuttons/

there are small borders (allthough this is not selected on grid alb element) these comes from width 33,3 % width if the width was defined as 33,33% it is not shown – but it might get in to problems on different browsers.
Easier to solve that problem is to give to the whole gird-row a the background-color of the middle column:

#av-layout-grid-1 {
    background-color: #a7a7a7;
}

Offtopic – one of my old requests – we need an ALB Element which reacts like a color-section ( as a container for different albs ) fullwidth but not pushing sidebar down but besides sidebar.
Or in other words an alb which reacts to the surrounding environment like a 1/1 container but inner behavior like color-section with drag and drop level 1 … that could solve a lot of layout idiosyncrasies

___________
this is a solution for a whole sticky sidebar ( aside ) …
now Ampower look here and scroll down the page – you see the sidebar is sticky as long the as possible – https://webers-testseite.de/buttons/
if you are interested in that solution i link you to that page here: https://kriesi.at/support/topic/an-actual-solution-to-a-sticky-column/#post-1124882

if you can’t transfer the solution to sidebar – i will tell you how.

this was the screenshot you gave me as you would like tlo have it:
my comment :

this has influence to the lines – add or change the existing rules to:

add new rules gave to you – change existing ones gave to you!

    ⇒         your last screencast

so all have right border – except the first row last column
where is that rule gone:

#header .avia_mega_div .units {
  border-right: 1px solid #010660 ;
}

On your screen there are no left lines to the first Columns

/* Nascondere per nascondere barra sinistra delle categorie OK */
#header .avia_mega_div .avia_mega_menu_columns_first {
    padding-left: 15px;
    border-left: none;
}

so you don’t like the border on the upper row right side – where is the new rule with the :not instruction:

#header .avia_mega_div :not(.avia_mega_hr) .avia_mega_menu_columns_last {
    border-right: none !important;
}

on insertion into functions.php : replace it with:

function insert_mega_div_separator() { 
?>
<script>
(function($){
$('<div style="margin-bottom:10px;" class="hr hr-custom hr-center"><span class="hr-inner inner-border-av-border-fat" style=" width:80%; border-color:#010660;"><span class="hr-inner-style"></span></span></div>').insertBefore('.avia_mega_hr');
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'insert_mega_div_separator');

this has influence to the lines – add or change the existing rules to:

#top #header .avia_mega_div > .sub-menu {
padding: 20px 30px 10px;
}

.hr-custom .hr-inner.inner-border-av-border-fat {
    border-top-width: 1px;
}

#header .avia_mega_div :not(.avia_mega_hr) .avia_mega_menu_columns_last {
    border-right: none !important;
}

Well, I almost had to help with that, didn’t I? ;)
now i’m out sorry – the next patient is already waiting.

btw: chrome on pc got the advantage that you can let the dropdown menus open when exploring them with a shortcut
believe it is F8

Safari on Mac did the trick without shortcut.

that rule here:

#top #header .avia_mega_div {
    max-height: calc(100vh - 200px);
    overflow: auto !important;
}

is for scrollable mega menu.
If the height of the mega div is more than the screen-height the overflow is on .
you can play a bit with the value 200px 50px seems to be enough

next comment to your navigation area.
if you get rid with :

.logo {
    margin-top: -100px;
    margin-left: -200px;
}

on Enfold there is on : Header – Header Behavior : ” Let logo and menu position adapt to browser window ”
Guess that this will fit better for your layout!

you could do this instead : https://webers-testseite.de/brighter/
if you like it – i’ll show you how to.

btw: after you have seen the test-page i delete your images!
it’s just to show you how it could look like.

well if you put in quick css this – you will see that it could work this manner you started.
But : there’s too much else wrong. Have a look at your navigation and the logo behaviour on small screens. Mobile devices so to speak.
I think one should first dedicate oneself to this topic. And then go to Borderstyling.

#av_section_2.test3 {
 position:relative !important;
 z-index:1 !important;
 background:transparent url(https://bright.itsomimedia.nl/wp-content/uploads/Rand_Bright_lichtgrijs.png) !important;
 height: 340px;
 margin: -300px auto 0;
 background-size: 100% 100% !important;
 background-repeat: no-repeat !important;
 background-clip:padding-box
}

https://kriesi.at/documentation/enfold/child-theme/

On default there is no child-theme present. But you can download a predefined one from link above.
A lot of those snippets here on board only work good with a child-theme.
You can update the parent theme without the risk of lost the made adjustments to the theme.

If you are not familiar with child-theme – read that carefully – and read my tutorial on that linked page above carefully too.

for the menu links : maybe you set for all links the content of the menu as alt attribute
this comes as the code above to child-theme functions.php:

function set_menu_text_as_alt(){
?>
<script>
(function($){
		$('#avia-menu .menu-item a').each(function(){
			var menuName = $(this).find('>.avia-menu-text').text();
			$(this).attr('alt', menuName);
		});
})(jQuery);
</script>
<?php
}
add_action('wp_footer', 'set_menu_text_as_alt');

try first without $(document).ready(function() this must work too

i have to see your site to give advice

and what kind of feature you like to have?

Sorry – i think this will work – but with my method you got that little message posibility in the gradient section and –
you can have all gradients (even images) in that second color-section without making allways another png file for it.

create your layout in this way:
you need for it two color-sections
the first color-section with your image and the content – i give a custom class to it:
background-with-content
the following second color-section – this is only for the gradient ( and maybe one short info text) with custom class:
background-with-message

the image and the gradient could be done in the alb element itself as you know allready.

put this to your quick css:

.avia-section.background-with-content {
  border-bottom-right-radius: 200px;
  z-index: 5;
  position: relative;
}

.avia-section.background-with-message {
  position: relative;
  margin-top: -200px;
  z-index: 0;
}

.avia-section.background-with-message .container {
    height: 300px !important
}

.avia-section.background-with-message .container .content {
    vertical-align: bottom;
    padding-bottom: 20px !important
}

looks this way: https://webers-testseite.de/rounded-background-gradient/

Edit: on very small screens you had to make a rule for that message in the gradient container.
padding-bottom or font-size etc.