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;}