I would like to change the background color of a single template on the dashboard. Unfortunately, when I use the code below, all template backgrounds are changed.
Can anyone give me a tip?
Of course you can. All the elements are styled with CSS and changing it is just to figure out the selector and then figure out styles for that element.
But for any case going down to specific use case, can't be advised without knowing exactly the element and current situation. If you have already some overrides, the conditions may differ from default and advises just may not work.
But best is to try to get current styles, modify them and figure out the selector using the browser dev tools.
And then, if thing don't go as you like, share the related things (make images, include the css rules) and describe your intentions.