We can add pages in site builder by creating tabs.It will contain the header and footer as of the site.We can add contents to it as HTML.
Steps
1.Navigate to list>website>tabs.
2.Click new and select new presentation tab.
3.To add contents you can can add it in in the message field by clicking its Formatted edit mode icon.
It can be added like in HTML pages .We can add styles and script in that also.
Additionally we can add contents using tags which is a subtab.
The page will contain all common styles.