Usually there are notifications in the Release notes and warnings are printed in the console about deprecated features. However there have been a few breaking changes in the past since Hugo has been rewritten a few times and it is still in v.0.59.1
You may edit Markdown content files online with the editor provided by gitlab.com
The themes used in that repo reside in this directory. So you can upload another theme for your use in your fork.
No there isn’t an online theme generator for Hugo, but there is the Hugo Themes Showcase featuring several themes that you may want to have a look at.