Hi @Walid Gharwal,
There is no such powershell script support to deploy a custom theme on SharePoint 2019. SharePoint Site Design and Scripts are only available in Office 365 modern UI. It's not available in SharePoint 2019 on premise version.
See note in this URL: https://docs.microsoft.com/en-us/sharepoint/dev/declarative-customization/site-design-overview
In SharePoint 2019 on-premise changing and applying of themes are quite same as SharePoint 2013 on-premise versions.
Please check below URLs for the same:
- https://docs.microsoft.com/en-us/sharepoint/sites/themes-overview
- https://docs.microsoft.com/en-us/sharepoint/dev/solution-guidance/use-composed-looks-to-brand-sharepoint-sites
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