Here is the document, not a stylesheet.
The styling is not complicated, but there are some CSS options I added for flexibility and to make it smarter.
As I advocate very often, do the styling inside the document while learning or developing it. This is faster and simpler as you see the changes immediately upon saving, no styling is ever cached, and there is only one document to deal with. Once the styling is working as desired, you can copy the styling to an external stylesheet (whether stored in DT or not) and link to it as needed.
Secondly, setting a default stylesheet isn’t the answer to all things Markdown unless you are only and always making documents with that particular styling. In this example, I flexed the headers in a specific way that would be undesirable in other documents, especially as prevalent as section headers are. A default stylesheet should contain only truly global styles. Again, if you’re only creating the same Markdown documents over and over again, the default stylesheet could be more complete. But if you’re using Markdown for many different purposes, it will be much smaller. Perhaps this forum post will clear some of this up:
And the document:
numbered-headers-final.md.zip (1.6 KB)
PS: This is not the only solution, but it works well.