deactivate responsiveness possible?

I think you are using Simple Custom CSS plugin and not Simple CSS plugin.

But anyways, I don't see anything wrong with that notation and I still don't see the code being added.
Did you add anything in line 1-28? Perhaps there is a syntax error in there that stops this block of code being executed?

Also if your ipad is 1536px in width on portrait, you should be seeing the desktop version?

Yes, I am using simple custom CSS and it works well so far. I will let you see all of my current css (incl. line 1-28). The other code is working fine. And no, I do not see the desktop version on the tablets portrait modus. That´s the reason for my inquiries :-)

Here is my complete css:
.main-title {font-family: "special elite", serif;}
.site-description {
font-family: "special elite", serif;
}
.site-info {font-family: special elite, serif; font-size: 10pt; }
.entry-title {font-style: italic; }

.page-title {font-style: italic; }

/*
Hintergrundbild für Blogkategorie: Aktuelle Reise

body.category-aktuelle-reise {
background-image: url("xy.jpg")
}
/*
Hintergrundbild für Blogartikel

body.single {
background-image: url("xy.jpg");
}
/*
Individuelle Formatierung des Widgets "Seiten/Sitemap" auf der Seite Sitemap
*/
#panel-1495-0-0-0 {font-size: 12px; color: #1a9604};
/*
verhindern, dass Sidebar in der mobilen Ansicht nach unten rutscht
*/
@media (max-width: 1025px) and (min-width: 600px) {
.content-area, .sidebar {
float: left;
width: auto;
left: auto;
right: auto;
}
.content-area.grid-65 {
width: 50%; /* Adjust to 100 - your sidebar percentage */
}
.sidebar.grid-35 {
width: 50%; /* Adjust to your sidebar percentage */
}
.separate-containers .inside-right-sidebar {
margin: 8px 8px 8px 8px; /* Adjust to separating space value */
}
}

Good night again

Right above the code I gave you, this line:
#panel-1495-0-0-0 {font-size: 12px; color: #1a9604};
should be
#panel-1495-0-0-0 {font-size: 12px; color: #1a9604;}

oha :-O
it works!!!! happy as can be. THANKS A LOT!

You're welcome :)