I downloaded presentations with nice themes I would like to use myself.
The only way I found was to delete all the content and write my own.
Isn't there a better solution than this one?
Please see chapter 2 "Using Slide Masters, Styles, and Templates" of the LibreOffice Impress Guide, esp. the stuff about 'slide masters': https://wiki.documentfoundation.org/images/5/5b/IG3500-ImpressGuideLO.pdf
more documentation at: http://www.libreoffice.org/get-help/documentation/