You can easily install a WordPress theme and activate it manually. Please, follow the steps below to do so.
Steps to install a WordPress Theme Manually
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Log in to the WordPress dashboard.
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From the Dashboard menu, go to Appearance and click on Themes.

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Click on Add New Theme.

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Choose Upload Theme.

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Click on Browse and choose the theme from your computer. Click on Install Now.
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It will take a few moments to install. After that, click on Activate.

Congratulations, you have installed and activated a new WordPress theme.