I got the subject bib-style at the homepage of our university. It should be used for theses. A comment inside the XSL-file indicates that the style comes from BibWord version 2.8. That's why I think my question would be in the right place here.
Since I use Microsoft Office 2013 I copied the XSL-file to
%APPDATA%\Roaming\Microsoft\Bibliography\Style\ and restarted Word afterwards, but that style doesn't appear in the reference style list in Word.
Has somebody an idea what could be a problem?