TechToBusiness

https://www.technologytobusiness.com/microsoft-sharepoint/add-spotheme-not-recognized

https://www.technologytobusiness.com/microsoft-sharepoint/hiding-default-site-themes-using-powershell

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

SharePoint Online will by default set the theme for your site. With modern sites, sometimes the theme is generated at random. If you were to create one new team site, you may see the them set to a Purple color, however, on your next site, you may see it appear as a yellow color. Sometimes, it is important for site administrators to create a consistent look and feel across all of the site content. This could directly impact the user experience and adoption of the site. In other cases, it may be important to meet the design standards enforced by an organizations Marketing Team. https://www.technologytobusiness.com/microsoft-sharepoint/change-look-sharepoint-online

https://www.technologytobusiness.com/microsoft-sharepoint/office-365-ui-fabric-theme-generator