Prevent Site Collection Administrators from adding other Site Collection Administrators via /_layouts/15/mngsiteadmin.aspx

I need to block SCAs from adding other SCAs via the /_layouts/15/mngsiteadmin.aspx page.  Changing COREV15.CSS file does not work if a site theme has changed since it no longer uses COREV15.CSS.  I think I can accomplish it using an event receiver but not sure which type of event receiver to use.  Microsoft.SharePoint.SPListEventReceiver, Microsoft.SharePoint.SPItemEventReceiver  or something else.  Any guidance is appreciated.

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Evy