As my notebooks get larger, I often try to organize them with titles/subtitles etc, structuring them as an outliner
This also helps me as I try to structure a notebook trying to produce an article, see this.
Unfortunately, I find that Mathematica's outliner capabilities are not so developed... a major source of irritation is that the markers indicating closed groups are very hard to see, and so small I often click on the wrong bracket level trying to open/close them. For me the ideal way would be a double-click on the block header to open/close it.
Also, it is not easy at all to take a block and shift it upper or lower in a hierarchy of levels.
Do you know if there is a way to enhance M's capabilities in this field? Maybe through some specialized stylesheet?
I see by the help you are kindly offering that there is indeed some way to enhance the outliner capabilities - so to rise the bar a bit, knowing that this is a typical outliner feature:
do you know a way to ask your notebook to "show everything down to level x" - that is e.g. keep anything under the level Subsection (or Subsubsection, or ...) closed?