createStyleMatrixContext() assumed that <dgm:style> contains
<dgm:fontRef>, but it contains <a:fontRef> instead.

This resulted in a 0 mnThemedIdx, which meant that since commit
89206c47 (bnc#862510: PPTX import: Wrong
text color inside shape, 2014-12-21) we ignored the theme color in
oox::drawingml::Shape::createAndInsert().

Change-Id: I81045fa58df5cb9e31a4ed3f1b1625224bcd06ad
Reviewed-on: https://gerrit.libreoffice.org/65938Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins