You can find our current documentation here: https://theme.co/apex/forum/t/knowledge-base-overview/287
Since X and Pro share many similarities, the majority of the articles apply to both themes. Specifically regarding stacks, these are found in both X and Pro. They're not technically sub-themes, but they have their own collection of pre-styled options. However, depending on how you're using Pro, you may never really encounter those pre-determined choices. For example, if you're using a blank page with a custom built header and footer, you're not going to see an obvious difference straight away. You may see the differences more easily when looking at something like blog posts, because they have their own distinctive look.
If however, you're saying that there is no difference at all anywhere irrespective of the stack selected, then that could be a problem for us to look into. That being the case, if you send us your login credentials in a secure note, we'll take a closer look at your specific setup. You can add a secure note by clicking on the little key icon beneath your post.
Also, just a final note regarding the documentation. We are currently in the process of a major revamp, which will make it a lot clearer to understand which articles apply to which theme. In the meantime, if you need any specific help finding something, just drop us a note here and we would be more than happy to help.
Hope this helps, and have a great day!