No, each Site (SPWeb object) has its own theme  property so that they can
be themed separately. I don't know of any out of the box way to push a
theme to all subsites. This could be coded using a Feature.

As I remember themes  only cascade if the Top level  site of the portal
has publishing turned on. If the top level site isn't a publishing site
(and if it's a migrated  SPS03 portal  then it probably isn't) themes need
to be set on each site.

I've just tried this on two Web Applications, one based on the
Collaboration Portal site template and the other based on Publishing
Portal, both have the Office Publishing Server Publishing
Infrastructure feature active, and have sites  and subsites where the
Office SharePoint Server Publishing feature is active. I then applied
a theme  to the Top-level site of the site collection, and themes  do
not cascade down.