Some CSS only works when local to the App

I have this CSS to tighten up the page a bit (less whitespace at the top):

.ViewerPageLayout__header {
  margin-bottom: 5px

If I put it in the app's custom CSS it works, but if I put it in the global Preloaded CSS, it does not. I do have other CSS that lives in the global space and seems perfectly happy.

This could be due to the ordering/priority of global vs app specific styles. If you place `! important` before the semi-colon on that style property, does it allow it to pass and be active?