FYI, once you have what you want. There is a template xhtml page you can set to be loaded by Sigil. It was added in Sigil 1.9.20.
From its release notes:
Quote:
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add ability for a user-template*.xhtml and user-template*.css be read from Sigil Prefs to create new empty xhtml and css files
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Where * is replaced by 2 for epub2 and 3 for epub3.