Themes
You can build your own themes by creating your own extension in which you can update constants, TSConfig, and partials and add your own assets.
The structure of the folders and files of an extension with its own Bootstrap and css files and assets will look like the image on the right. In the example, the partials Content/Main/Cards.html and Page/Header.html overwrite the ones from the extension Das.
In order to overwrite assets and the path to the partial folders, the files mydastheme/Configuration/TypoScript/constants.typoscript and mydastheme/Configuration/TypoScript/setup.typoscript should contain something like in the images on the right.
Once the extension is installed, include its TypoScript template.
The themes Quartz, Lux, Slate and Vapor have been taken from Bootswatch, to be used just as examples.