[Confluence] add default EPG background color for genre strings

XBMC has a special genre type EPG_GENRE_USE_STRING (0x100) that allows PVR addons that do not provide GenreType/GenreSubType id's to pass a genre string instead.

It turned out that that the corresponding skin color picture was missing resulting in a transparent EPG timeline as shown here:
http://www.argus-tv.com/forum/viewtopic.php?f=60&t=3968&start=30#p27546