You can use a selector like [id^='page-members'] or #parent-page__members
In this example there is a database page called members with pages in it.
The first selector will select any page with the id that begins with page-members - this will include the members gallery
The second one will select any page that is a subpage of the members page - this will only select subpages of the members page
Example One
Just say you want to format callouts on the members page. Here is the general code for that.
.notion-callout {
color: red !important;
}
To make this code only apply on the members page, add the ID tag.
[id^='page-members'] .notion-callout {
color: red !important;
}
Example Two
If you would like to target all the subpages of members page, then:
#parent-page__members .notion-callout {
color: red !important;
}