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You can do this by changing the database with phpMyAdmin or the equivalent.
In the database table xoops_config, find the row with conf_modid = 0 and conf_name = "theme_set", and change conf_value to "default".
For the template set, look for conf_modid = 0 and conf_name = "template_set", and change conf_value to "default".
You may want to backup the database first as a precaution.