Set Theme as Default
Say you spent some time designing your own Theme. You
selected your Color palette, Fonts and Effects. You can save your
Theme as the Default theme used with each new document that
you create.
Try it: Set Theme as Default
Go to Page Layout -> Themes.
Go to Save Current Themes.
You will be prompted to enter a name for your new
Office theme (*.thmx).

Memo to Self: You can also use
this menu to Restore the Theme to one of the built in templates.