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?
File --> Templates --> Save as Template
. Give some name to it and save it ( I used the predefined folder "My Templates")--> slide design --> Load
and select the template you savedView --> Master --> Slide Master
There you can again right-click and change the slight-design of the current Master.Tested with LibreOffice Version: 4.2.8.2
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/
when you have the desired template open you should go to File --> Templates --> Save As Template...
Click on save, enter template name and validate... When you wish to use this template any other time just do this; File --> Templates --> Manage
Select the Presentation tab and choose the template you saved. Hope this helps you!