Creating Custom Site Themes Using Powershell - SharePoint Online

As SharePoint Online was initially rolling out the modern interfaces for mobile friendly site pages, we quickly found out that we lost the ability to customize these pages in a way we were familiar with. 

With the introduction of custom themes, we now have the ability to perform basic customization of site pages in terms of color and font. Microsoft has provided ways to quickly generate new themes and deploy them to our sites. We can use SharePoint Online Management Shell to perform these administrative actions.

https://www.technologytobusiness.com/microsoft-sharepoint/create-sharepoint-theme-management-shell