Well, I figured out one thing-- I think!

I copied a small data set into a new workbook, created a graph, and
then changed the colors using Excel Preferences Chart Fills and Chart
Lines.

I then saved this as a template named "Workbook" in the Office Startup
Excel folder replacing the default workbook template.

Of course, that means every new document comes up with the small
dataset and chart. So I reopened the default Workbook and deleted the
data and the chart and re-saved it. Voila! It seems that I now have
chart colors and bar fills in the colors I'd prefer.

But that still doesn't get rid of the awful grey background. Note:
when one changes the Chart Fills colors that really is changing the bar
fills, not the actual background fill. (The Excel lingo should be
changed in that box.)

So, I've solved, I think, the colors problem... but how does one change
that ugly, dark, miserable grey fill?