Hi,
I added the css to your child theme style.css at the end directly, and it seems to be working now.
Please clear your cache and check.
For some reason the General Styling > Quick CSS field didn’t update your css, but this may work it’s self out with your caching off.
I recommend keeping your caching off until you are done building the site, also check your General Styling > Quick CSS field for anymore rules that are not being applied, as you may want to move them to your child theme style.css
I notice that your child theme seems to have a copy of every file that makes your parent theme, is this by design? I’m used to only seeing modified files there. I wonder if this is a factor in General Styling > Quick CSS field not working as expected.

Best regards,
Mike

Hey Dialogo2017,
Yes we can

.main_color.iconbox_top .iconbox_content {background: blue !important; }

Do you want to change the thickness of the border of the icon box?

Best regards,
Mike

Hey pspsolutions,
I see you are using Enfold Version: 3.4.3 with WP 4.9
Please try updating to Enfold v4.2

Best regards,
Mike

Hi,
I disabled your caching plugins, & cleared them a few times and added:

#wrap_all #main .main_color .template-page .entry-content-wrapper  h2 {
    text-transform: none!important; }
strong a,.main_color a {color: #67b5a5 !important; }

which should have worked, but didn’t.
Can you see if your host is also caching?
If you want me to enable your caching plugins I will, but thought this would be a good test.

Best regards,
Mike

Hi,
Try using WP Reset and changing PHP Post Max Size & PHP Max Upload Size, and trying again.
http://www.wpbeginner.com/wp-tutorials/how-to-increase-the-maximum-file-upload-size-in-wordpress/

Best regards,
Mike

Hi,
Glad we were able to help, we will close this now. Thank you for using Enfold.

Best regards,
Mike

Hi,
I have tested the plugin on a clean install of Enfold and it doesn’t seem to be compatible. Sorry but not every plugin will work with Enfold. Have you considered using the built in video element? Or perhaps a different plugin will have the features you are looking for.

Best regards,
Mike

Hey Stefanie,
Here is instructions for creating a lightbox for mailchimp form

Best regards,
Mike

Hi,
Thank you for sharing your solution, shall we close this now?

Best regards,
Mike

Hey seifenblasenfabrik,
Please see screenshot in Private Content area of the errors your site is giving.
The first few are for the old media player in WordPress that are not a part of v4.9, others on WP forums report that during the update these should have been replaced/removed.
It is recommended to reinstall WP, yet I see you are using 4.9_en_DE_formal and this offers another issue as in v4.9 the language DE_formal is giving errors, to solve this it is recommended to switch to the other one. Do you have a managed host that can reinstall 4.9 DE regular or EN and set your language after? Or try changing your language and reinstalling WP.
The other errors are from your SEO plugin, but it also might be related to the language.
Try disabling your plugins before your reinstall, and don’t enable until you have tested the builder.

Best regards,
Mike

Hi,
Try this code in the General Styling > Quick CSS field:

#top.home #header #text-5.widget {
    position: relative!important; 
}

Best regards,
Mike

Hi,
Try this code in the General Styling > Quick CSS field:

.page-id-23571 .main_color.container_wrap_first .content {
    padding-bottom: 0px!important; 
}
.post-entry-23571 .el_before_av_tab_section {
    margin-bottom: 0px !important; 
}
.page-id-23571 #av-tab-section-1 .av-tab-section-tab-title-container {
    padding-top: 0px !important; 
}
.page-id-23571 .av-tab-section-inner-container .el_after_av_tab_sub_section,.page-id-23571 .av-tab-section-inner-container .el_before_av_tab_sub_section {
    padding-top: 0px !important;
}
.page-id-23571 .av-tab-section-tab-title-container .av-inner-tab-title {
    width: 120px!important; 
}

please see screenshot in Private Content area

Best regards,
Mike

Hi,
The builder that is built into Enfold, Advanced Layout Builder (ALB) uses many custom codes (shortcodes) that are always activate whether you are using the ALB or not, using other builders may accidentally trigger these causing undesired effects, such as in your case. I imagine anything your want to build with Elementor can be done with ALB, and more. We are always happy to help you achieve your goals, feel free to open a new ticket should you need assistance with using the ALB.
As for your current page, please disable Elementor and try to start a new page using the ALB, and you should find that the undesired social icons will not be there.

Best regards,
Mike

Hi,
Your PHP Version is still showing as 5.3.24, you can see this by going to LayerSlider > System Status > Server Settings
and still a error 500 (Internal Server Error)
Please check your .htaccess file for the PHP version being declared, this is sometimes the case.

Best regards,
Mike

Hi,
The Elementor Builder should not be used with Enfold, it must have added code that is similar to our social icon element code.

Best regards,
Mike