þ ð These are 2 Icelandic letters not in your Special Characters file. The first is the letter for the soft “th” sound (thin), and the second is the letter for the hard “th” (this). Any chance they’ll be added any time soon?
To paraphrase what @manj_k says in the answer below, this is a font-dependent matter. Any font with these characters will allow selection as shown.
Do you refer to these characters, and to this dialog?
(LibreOffice 4.0.2 · Font: Liberation Sans · Subset: Latin-1)