How to change the header and edit other Single pages of Portfolio items settings?
On Appearance > Customize > Portfolio panel there is a Settings Page Slug field. This field should contain the slug of the page which serves as a template for all portfolio items. All single pages of portfolio items will inherit settings and shortcodes of the page referenced via this field.
In order to change elements on single page of the portfolio item, please locate the page with the slug entered in Settings Page Slug field and edit it.
For more info and detailed instructions please refer to our Online Theme Documentation.