xmlui: Re-work XMLUI themes for new thumbnails

When using filter-media with xpdf we can get automatic thumbnails
at a much higher resolution (300x300px right now), so we need to
hard code a sensible thumbnail size in the XMLUI so the browser
scales it down (otherwise it uses $thumbnail.maxwidth from dspace.cfg
and the XMLUI gets ugly due to the xsl performing CSS math based on
that value).