Currently moving to SP2019 on premise. I want to brand a new modern site. I would like to change the theme colour - which I guess is a new theme, add a logo - I don't think this is a big deal, change the default font and hide the SharePoint text at the top. I believe the modern pages are not customizable but I do believe the themes can be customized? This platform is completely new to me - any advice would be appreciated.
In SharePoint 2019 on-premise, creating and applying custom themes is quite same as other SharePoint on-premise versions.
Please check below URLs for the same:
To Hide the SharePoint default text or anything, you can use SPFx Application Customizer.
Currently in SharePoint server 2019 modern site,you could change the logo,customize the theme.please refer to the following article for details:
Change the default font and hide the SharePoint text at the top will use SharePoint framework extension application customizer to inject the CSS to your modern SharePoint pages.
Following sample tells you how to inject the CSS: