Have finished, but only bit I can't parse is in 3 Down, the clue for which is:- "Prevailing idea of twelve points in article". The answer is THEME, bit I can't see where "twelve
points" comes into the reckoning.
That's a different usage of "em" than the one I know, cerasus. I know it as a space the same width as a lower-case "m", so it varies according to the font size.
An em space is a term derived from the width of the capital "M" on a metal character block used with early print presses, similar to the blocks shown in the picture on this page. Today, an em space is used to define the current point size of the font