Customize menu in Omeka S

Dear all,
We would like to add a background-image to the menu of one of our Omeka S exhibitions. When we add the changes, it changes for all the exhibitions.
How can we avoid this? Which code do we need to change? We’re using the default theme.
Thank you in advance,

Are your different exhibitions different sites? Or different pages/parts of one big site?

If they’re different sites, simply making a copy of the theme and and applying your changes to the copy would let you only pick that new theme on one site and thereby have the changes only take effect there.

They are different sites. Thank you for your answer!