Thank you very much.
So I tried it and it looks great...BUT.
There appears to be a weird bug. (first printscreen)
When I applied the code to my custom CSS setting, it went exactly as expected, except for some reason it shrunks "menu/topbar" as well.
Then I tried how it looks on a different pages than Home/index and found out that it moves every small aspect as well, and when I scroll down, part of the menu is missing as well as the right navigator button....Like if the page moved to the right.
As I was looking for a reason, I found out that the only thing that seems to affect it is in Settings -> styles > layout - "wide topbar" is checked/ or unchecked.
Because when I unchecked it, only the main body of the page changes its width, while everything else works normally, and I presume that is how it should be. I tried a few percentages ( even 50%) and when "wide topbar" is not checked, it affects only the body where the gallery is, and nothing more and gallery "shrunkes" nicely.
I tried to use the code to one page only (manage -> index page -> page -> advanced -> page css), just to be sure it is happening only to that page, and it does.
( I used page background color plugin so the changes are more visible)