Hi ninoparisi
On the post which you referring it was asked to add custom code to custom.css file
Then add to your custom.css file in this folder /templates/ja-fit/css/
.t3-header.affix {
position: fixed;
top: 0;
z-index: 10000;
width:inherit;
background:#000;
}
@media (min-width: 992px) {
.affix .t3-mainnav {
top: 90px!important;
position: fixed;
width: 1600px;
max-width: 100%;
}
}
If you do not have a custom.css already then create a new one.
I do not see any custom.css file added to your website , please add it and then verify again.
Regards