Hi there,
so that CSS is being applied to all pages, so if there are multiple pages that you would like to remove those margins then do this instead:
1. remove the CSS you have.
2. Appearance > Elements > New Hook and give it a title.
3. Add this CSS as the content:
<style>
@media (min-width: 415px) and (max-width: 1480px) {
#content {
margin-left: 20px;
margin-right: 20px;
}
}
</style>
4. On the display rules tab set the pages / archives / posts etc. where you want the code to work. Or if its simpler use the Entire Site rule and Exclude the locations you want to remove it from.