Can I have a Writer template document which serves the basis for the other documents styles - not only for the new ones (if it is selected as default), but for all files that “use” the template, just like a .CSS file continuously auto updates the look of .HTML documents that use it?
Templates are not used in documents, unlike style sheets. Each document has its own copy of style sheets, which changes are reflected continouosly, but as you see, this style sheet is local to the document. Templates are used to create documents, i.e., copy all their content (including, but not limited to, styles).
Having said that: there is a mechanism to synchronize styles with templates used for documents creation; see this answer for details.
I think the linked answer is too long… so, it’s not helpful
(I don’t want to read the story of someone who works for someone who has an eye problem…, bur really, & no offense)
your explanation is very clear,
however, in the end you refer to a “mechanism”… and a long story 
instead of answering with a clear YES or NO (beyond the very cool explanation you gave)
so, the question remains, will documents update once the template they are based on gets updated?
my answer to this is “yes”. or “yes, unless there is some glitch”
I remember that in normal cases it worked like this…
I modified the template
opened the doc,
and changes were implemented
but glitches happen to templates, especially lately…
If you modify a template, then when you open any document created from the template, you will be prompted to confirm changes. If you confirm, all styles inherited from the template will be updated. Is that what you need?
As described in the answer I mentioned, this has some preconditions to work.
thank you gabix for making this clear!