There is no control over the background of equations.  There might also be an
import problem.  This is perhaps related to
http://www.openoffice.org/issues/show_bug.cgi?id=3504 .

In slide 9 of http://www.writing.eng.vt.edu/speaking/slide_template.ppt the
equations are imported with transparent background rather than white.  The
result is that underlying text shows through.  Using MS Powerpoint the equations
have solid white backgrounds and the underlying text is not visible.  There is
no Impress control over the background for equations.  All equations are created
with transparent backgrounds and black text.

Also in slide 9, the equations are imported, but are marked as read-only.  This
may be related.