Find the Ableton 10 application in your Applications folder, right click on it and select Show Package Contents. Then inside the app, head into Content > App Resources > Themes and drop the theme in there.
Drop the theme into C:\ProgramData\Ableton\Live 10 Suite\Resources\Themes.
Go to Ableton and open the Preferences window, if it's open already close and reopen. The theme will then be under the Look/Feel tab.
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