Well any default theme won't exist in sdk itself. In prior versions of sdk (sdk 3.x and below), acs and share used to run in embeded tomcat within sdk, then it was possible to get ootb items within sdk.
SDK 4.x based out of docker containers so you have to look for the default themes in share container at this path:
You can copy the default themes from there and create your custom theme by taking reference from there.
Or if have share war file, you can extract and get the default themes:
War download link:
Follow these documentations and develop your custom theme:
Have a look at this thread, it may be helpful:
You can also find some theme demo project here for reference: https://github.com/abhinavmishra14/share-theme-demo/tree/master/share62-theme-demo
(ACSCE, AWS SAA, Azure Admin)