How can I hide a thumbnail?

I didn’t really read the code but if I understand correctly, you should be able to hide it with CSS (assuming the grid-wrapper div is inside the single-wrapper div):

#single-wrapper #grid-wrapper {display: none;}

and if that doesn’t work, try:

#single-wrapper>#grid-wrapper {display: none;}

Put them in your CSS file.

I really appreciate your reply.
but it doesn’t work.

I don’t think the ‘grid-wrapper div is inside the ‘single-wrapper’ div.
These two are separated.

Yeah, I see grid-wrapper isn’t inside single-wrapper. If you are able to edit that file, find where <div id=”grid-wrapper”> starts and add class=”hidden” to it, so it would come out to:

<div id="grid-wrapper" class="hidden">

Then in your CSS put:

.hidden {display: none;}

..
I’m so sorry. It doesn’t work.

A Thumbnail is always showing in this code.
so..

I want to write a code like this.

“If there is a selected post, a thumbnail is not showing.
and
If there isn’t a selected post, a thumbnail is still showing.”

How can I do that?
I need your help.