“pretty” does not make an app more powerful or functional.
Currently, no but there are also ways you can add your own, and has been covered in the Forums before (More Markdown Support)
If the Sorter isn’t running, a Take Note window will appear when you use Tools > Take Note.
Beyond cosmetics, are there functional issues that keep DEVONthink form being an effective note-taking app? (And again, I ask as someone who uses it every day - especially with Markdown but also tons of plain text too.)