Background (answers to your questions follow):
Word 2007 introduces several very powerful features that let you easily
create professional-looking, aesthetically appealing documents; these
features include document themes and Quick Style Sets, each of which allows
you to change the look and feel of an entire document (text and graphics)
with just a few clicks.
There is one active document theme (Office by default) and one active Quick
Style Set (Word 2007 by default) for each document.
Each document theme (there are 23 built-in) consists of a set of theme
colors, a set of theme fonts and a set of theme effects. You can mix and
match sets of theme colors, theme fonts and theme effects from different
document themes. They can be applied from the Page Layout tab, in the Themes
group. Live Preview is available.
Each Quick Style Set (there are 11 built-in) consists of 25
aesthetically-coordinated paragraph styles, linked styles and character
styles. Each of the 25 styles has the same name in each set. A different
Quick Style Set can be applied from the Home tab, in the Styles group, by
clicking Change Styles, pointing to Style Sets and selecting the desired
Quick Style Set. Live Preview is available.
Your 1st and 2nd questions:
The active set of theme fonts (a font pair: +Body and +Headings) determines
the font (Calibri, Times New Roman, etc.) directly or indirectly for every
style in every Quick Style Set. The default Office theme sets Calibri as the
+Body font (and Cambria for the +Headings font).
No Quick Style in any Quick Style Set specifies a font. They include +Body
or +Headings (all 9 Heading styles in most sets) in their definition or
inherit the font since they are based on the Normal style.
Your 3rd question:
To have Times New Roman as your default +Body font, click Fonts in the Page
Layout tab, in the Themes group, and then click Classic (+Body is Times New
Roman and +Headings is +Arial) as the active set of theme fonts.
There are other ways to do this but this method preserves the power of
document themes and Quick Style Sets. I would strongly encourage you to
experiment with changing document themes and changing Quick Style Sets to see
how they work seamlessly together to help you create designer-quality
documents.