When designing Custom Pages there are some parameters that will need to be considered. For example; the ability to design content within a page on your platform does not include all the platform navigation and other hard-coded design, which will stay uniform across your platform.
As a general rule a Custom Page refers to the space underneath the navigation menus, page headings, search box, and actions list (such as Edit, Download PDF, etc).
The term Custom Page also does not refer to such pages as the log-in or registration page (although some customization such as adding a background image for this can be done within Admin > Templates).
Finally, within the actual Custom Pages normal CSS and HTML rules usually apply, with some restrictions.