1)  I do essentially all my notebooks in a single default format or
stylesheet, which I'd like to edit in some trivial ways (e.g., change
the working Text font to Helvetica.

What's the recommended procedure for doing this?  Can I (should I) edit
the Default.nb file directly?  Or better to create a new myDefault.nb
style?  How do I do this, and where do I put it?

[And is there somewhere I can read about this (or actually get a step by
step description of how to do it) without getting lost deep in a
sequence of infinitely cascading "Help" files?]

2)  As a trivial add-on point: The Format >> Style menu brings up a
submenu that shows you the Style you (or someone) selected for a given
cell (the selected Style is checked).  

The Format >> Stylesheet menu brings up a submenu that *doesn't*
indicate the Stylesheet that you (or someone) selected for the selected
notebook (the selected Stylesheet is *not* checked).  

Why not?

Piece of cake dude:

Select Format/EditStyle sheet.  You'll get a blank "private style sheet".  From the left menu that says, "choose style to modify", select Text.  You'll get a skeleton override for Text at the bottom.  Open up this cell with Shift/control E. It'll look something like:

Cell[StyleData["Text"]]

See, it's just a skeleton.  Now add, FontFamily->"Helvetica" to it like this:

Cell[StyleData["Text"],
FontFamily->"Helvetica"
 ]

Close the cell with another Shift/Crt E.

Now save it as something like "myNewDefault". Make sure it's saved in the StyleSheet directory that Mathematica expects to find stylesheets.  Now may need to exit Mathematica for it to show up in the list of styles.  Do that, bring up Mathematica again, should be in the list.  Choose it and your Text format should be Helvetica.

It is a bead idea to edita any of the stylesheets that are shipped
with mathematica.  You sould definitly create a stylesheet of your
own.

Here is a simple way to do this.  I am sure that there are many other
ways...

Chose the base stylesheet that you want for this notebook, for
example, Default.

Open a new notebook and choose "Edit Style Sheet from the Format Menu.

There is a resulting notebook that you can create Private style
definitions for the original notebook.  Call this the "Private
Notebook."

Note that there is already a cell that says "Inheriting the base
defiinitions from the stylesheet "Default.nb" (for your shoice of
Default)

Now make the changes  that you wish in this location.  For example, if
you want to change the "Text" style, choose "Text" from the pulldown
menu that says "Choose a style to modify".

Make the changes to the resulting "Text" style cell.

Make any other style changes that you wish to other styles in this
way.

Now copy all of the cells in this notebook including the one that
says  "Inheriting the base defiinitions..."

Open a fresh new notebook.

In this notebook execute the following

nb=EvaluationNotebook[].

Now delet this and its output from this notebook.

Go to the "Private Notebook" and select all of the cells that you
placed in there and copy them.

Now go to the notebook nb and paste these cells there.

In a separate notebook (not in the notebook nb) execute the following:

SetOptions[nb,  StyleDefinitions -> "StylesheetFormatting.nb"]

Now go back to nb and save it to a locatoin on your hard drive=,
giving it a sensible name such as MyStyles.nb.

Now go tot he Palettes menu and choose "Install Palette..."  (yes,
yes, I know...  it shoudl also be on the Format StyleSheet Menu...).

In the resulting dialog choos "Stylesheet" from the pulldown menu, and
choose ""File.." from the source pulldown menu.  In the Install Name
field give youe style a name.  And, finally, choose the radio button
for the Default Installatoin Directory.  Now click "Finish."

The stylesheet will now appear on your Format> Stylsheet menu.

As for your point (2).  No the stylsheet that you are using for a
notebook is not indicated in the Stylsheet submenu.  I think that this
is an oversight and (I think) that I reported it a long while ago.

In A WorkLife FrameWork, the "NotebookS tyles" Palette does indicate
the current stylesheet of a notebook.

Give  A WorkLife FrameWork  a try! There is a 15 day trial license and
I'm happy to extend it once if the 15 days isn't enough.

In fact everyone on this group (who hasn't purchased a license
already!) should give it a try!

The next version will contain tools to create documentation for the
version 6 Documentation Center from convetional Mathematica
notebooks...

--David
http://scientificarts.com/worklife
A WorkLife FrameWork
E x t e n d i n g MATHEMATICA's Reach...

On Oct 1, 4:58 am, AES <sieg...@stanford.edu> wrote:

After writing the previous detailed answer, I realize that I should
just add a small suite of tools to A WorkLife FrameWork that automates
the creation fo new stylesheets.  So, I will do this...

--David
http://scientificarts.com/worklife
A WorkLife FrameWork
E x t e n d i n g MATHEMATICA's Reach...

On Oct 1, 4:58 am, AES <sieg...@stanford.edu> wrote:

Ok, and update to my earlier set of instructions that simplifies
things.

When you have created the new style definitions in your original
notebook's "
edit StyleSheet"  working area, select the cells that are there
(including the first one that says "Ineriting base definitions...).

Next copy these to the clipboard via File>Edit>Copy (or the keyboard
eauivalent).

Now go to File>Install...  to bring up the instll dialog.

Choose Stylesheet from the first menu.

Choose "From Clipboard" from the second menu

and type a name for your stylsheet in the input field and then click
Finish.

Voila, the stylsheet now exists and is in your Stylesheet submenu.

This obviates all of the scratch notebooks that I used in my previous
post.

--David

On Oct 1, 4:58 am, AES <sieg...@stanford.edu> wrote:

Unless one is going to do something really fancy, a safer way to modify
a cell style such as Text is just to use the main Mathematica menus to
format it directly.

So in this case, after you've select Text as the style to modify, select
the entire cell that appears in the Private Style Definitions notebook.
  Then from the main menu select Format > Font... and from the dialog
that opens, select Helvetica.

This way may be more tedious that editing the "expression form" of the
cell, but it leads to fewer syntax errors.

--
Murray Eisenberg                     mur...@math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

Thanks very much to David Reiss (and "chuck" and "Murray") for responses
on this.

Having now successfully done it, seems to me the way to continue doing
it is:

1)  Create, modify and save "myDefault.nb" _once_ using any of the
approaches alluded to above.

2)  Use the technique given by David Reiss in another post to add
myDefault.nb as a sticky item in the File >> Open Recent menu.

3)  If and  when further changes to default stylesheet are desired, just
open myDefault.nb from there and do standard stylesheet editing.

[and, I guess, 4) Restart.]