This stylesheet contains 3 occurrences of
!important. You could try surgically removing them. I've had enough trouble with this in the past that I now always remove them before sending to a Kobo. If the css is well-designed it should never be needed anyway (IMO

). I've had numerous occurrences of an epub displaying on a Kobo as if none of the css was being used, i.e. everything left-aligned, no paragraph indents, 'blank line' between every paragraph. When I've opened up the epub to investigate it has always contained at least one !important. Removing them has always fixed my problem.
BTW, in addition, there doesn't seem to be a space before !important which I would have thought is an error, e.g.
Code:
div.abbildung.with_thumbnail p.link_toFullsize {
text-align:left;
text-indent:0!important;
}