[philja]

Should be possible by creating a template and storing it in your Sigil Preferences location.

Full details are in the Sigil User Guide but roughly speaking, you edit the existing new xhtml page to your liking and then save it as user-template2.xhtml (for epub2 files) or user-template3.xhtml (for epub3 files) to your Sigil Preferences location.

You can do the same for a custom CSS template file.

To find your Sigil Preferences location, in Sigil go to Plugins > Manage Plugins and click the button, "Open Preferences Location", at the bottom left