Why does the theme of secondary site load the primary one in WordPress Multisite? [closed]

In the multisite backend dashboaord, in the top left corner, you have a My Sites link. Hover your mouse over it, and a dropdown will come up. The first option is Network Admin; click on it.

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Once you are at the Network Admin section, you can manage your themes and sites.

You can add new themes and list all existing theme.

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Note that if you click on Installed Themes, you will also have an option to Network Enable a theme for all sites.

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In your case, I would NOT enable any theme for all sites. I would leave the theme disabled, then I would click on Sites, and you will see all your sites listed.

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For each site, go to the Theme tab, enable the specific theme for that particular site.

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Once you do so, you will be able to restrict the theme to the site.