Pages does not have a provision for changing the default viewed page color, without also changing the color of the printed/saved/exported document to yellow too.
The free LibreOffice has very flexible appearance controls in its Preferences. There are 9 shades of yellow that one can set the default document view too. Saved or exported documents have the traditional white background. There are 5 other icon styles available from the Tango style shown in the following screen capture. LibreOffice is an MS Office replacement suite with highly versatile document interchange capability. There are PDF manuals for the individual LibreOffice applications, and the suite installs in one location, the Applications folder.