To make a theme the default, in the Admin CP go down to Settings > Basic Settings and this will open up a page for Global Site Settings. Write the name of theme in the in the Default Theme input box and hit 'Change Global Settings' to save it.
If you'd like to remove people's ability to select other Mysidia themes, in the Admin CP go to Links > Edit Links and it will open up the Site Links Manager. All you have to do is remove the link that would take visitors to changestyle.
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