I previously considered PRSPlus, but decided not to mess with the original Sony FW. I think I would also lose my built-in dictionaries on the PRS-350.
Your earlier suggestion appears to work, i.e replacing this
<BODY style="FONT-WEIGHT:normal; FONT-SIZE:small; MARGIN-BOTTOM:245px; LINE-HEIGHT:normal; FONT-FAMILY:GeorgiaEInk15" text=#000000 bgColor=#ffffff>
Why that code is there if it's not needed, I do not know. You said it appeared to be redundant, and that's the way it looks. (Except the font must fall back to some default, since that code contained the only font reference in the htm document.)
I don't need to switch fonts in the middle of a book. I just like to decide on a font before beginning a book, and want the ability to quickly use Calibre to convert the original htm to epub with the selected font.
So now I can do this:
-Clean up book in RTF.
-Create book in BookDesigner and output to htm.
-Edit htm in Notepad to adjust the single body tag as above.
-Place htm in Calibre for conversion using Extra CSS for font designation.
I have saved Extra CSS statements for each font that I like. So it's only a matter of copying one of them to the Calibre conversion Look & Feel screen and converting to epub. (The rest of the conversion settings are already saved in Calibre Prefs.)
Works great for a guy that does not know html.