Brand modern SharePoint Online sites
With SharePoint Online modern sites the branding options that used to available with Classic sites were removed, however Microsoft introduced new alternatives to apply you company brand to SharePoint.
The new theming experience allow site owners to apply themes to all modern pages in the site collection, to deliver new engaging and familiar looks.
By default, there are 8 themes available but you as an Administrator can create and deploy your owns, in this article you will find instructions on how to achieve it.