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Where did you download the theme from?
I am aware of Yogurt "module", but not of Yogurt "theme".
If you have the Yogurt module, this is not maintained anymore, so I would suggest using
SmallWorld module.
If indeed you have a Yogurt theme, you can change it using phpMyAdmin, by changing the theme name from "yogurt" to "default" or "zetagenesis" in the "config" table in the "theme_set" record.
Also, you can use the Installation and Operations Guides
from here. The video is outdated.