You are welcome :)

Did you lookup 'default font' in Word help?

First, here's a little background on how the (Body) and (Headings) fonts
function. Unlike past versions, in which you could specify a single default
font, you can now specify two default fonts, one for (Headings) and one for
(Body). If you use the (Body) and (Headings) fonts in your document then
modify the Theme, specifically the Theme Font set, the (Body) and (Headings)
fonts will automatically update to those defined in the Theme Font set.
While you can set a default font, other than (Body) for your document or
template by modifying the Document Defaults, this will only apply to the
(Body) font and not the (Headings) font. If you want to create documents
that can quickly swap fonts to adhere to another specified formatting then
it's best to modify the Theme Fonts instead. This also allows you to use the
same font for both (Body) and (Headings) if you'd like. (Also note you can
disable the ability to swap fonts in a document if that is a concern.)

You'll find Theme elements on the Page Layout tab, in the Themes group.
Alternatively, on the Home tab, in the Styles group, click Change Styles.
For either method, click Fonts and either select another set of Theme Fonts
or create a new Font Set using the link at the bottom. Doing so will modify
both (Headings) and (Body) used by styles that utilize the (Headings) and
(Body) fonts in your document. If you want your selected Theme Font set to
be the default for new documents based on the document template, then from
Change Styles, click Set As Default. (If your document is based on a
template other than the Normal template then this command will read, Set As
Default for <Template Name>.)

If you want to see if modifying the Document Defaults is what you are
looking for then display the Styles task pane, click Manage Styles at the
bottom, click Set Defaults, and modify the Font. Again, note if you set your
default font to something other than (Body) then your new default font will
not swap if the Theme Fonts are changed but those styles that use (Headings)
for the font will swap if the Theme Fonts are changed. If you want to
prevent your Headings from changing then you need to modify the styles that
use the (Headings) font and define another font. However, I've found it's
far easier and faster to modify the Theme Fonts for a document/template and
disable font switching if necessary.

As for documentation, so far I don't know of any detailed resources
available online. There might be in a few books that are available that
contain the details that you are looking for - I'm certain one does, anyway.
;-)

Please post all follow-up questions to the newsgroup. Requests for
assistance by email cannot be acknowledged.

~~~~~~~~~~~~~~~
Beth Melton
Microsoft Office MVP

Coauthor of Word 2007 Inside Out:
http://www.microsoft.com/MSPress/books/9801.aspx#AboutTheBook