In order to change a theme, you just have to:
- Authentify for your new blog at http://myblog.ro/wp-admin with the username admin and the password you chose during command.
- Select from the administration panel, from the Tools section, the option Take over
- Go to Visual properties, look for your favourite theme and activate it.
Some themes call for a menu, in the administration interface, which allows the upload of a header picture or the change of certain visual parametres.