Take a look at the comment at
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which has the latest commands related to themes (actually, as of 3.0, but they seem to work with Drush 4.x as well).
- Disable theme: <code>drush -l
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pm-disable theme_name</code>
- Enable theme: <code>drush -l
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pm-enable theme_name</code>
- Change theme: <code>drush -l
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vset theme_default theme_name</code>
- Change admin theme: <code>drush -l
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vset admin_theme theme_name</code>