It's certainly do-able, but not trivial if you're not fairly comfortable
with VBA coding. The lines of the displayed document are available through
the Page.Rectangles collection. This is a somewhat mysterious bit of object
modelling the describes the physical rendering of the document as a
collection of rectangular areas (header, footer, columns, lines, graphics,
etc). Your challenge is to find the lines, and successively highlight
them -- eg by positioning a color rectangle under the line in question.
Don't interfere with the text itself, eg by bolding it, because that will
sometimes cause the page to reformat, since bold text is wider than normal
text.

Hi Bill,

see: http://word.mvps.org/faqs/macrosvba/AppClassEvents.htm

You may use a WindowSelectionChange-event
and highliht a line, like:
Selection.Bookmarks("\line").Range.HighlightColorIndex = wdYellow

The more difficult part would be, to remove the highlighting
again after the windowselectionchangeevent.

If you really want to have such a function,
show us, how far you get without detailed help.

Can all be done, more or less.