From what I see, the tango icons are taken from the Tango project.
From the tarball I downloaded from tango.freedesktop.org, the license
is CC-SA-2.5.
One problem with it is that it is not compatible with GPL (AIUI,
because it poses additional restrictions).
Another -- that CC-SA requires that derivative works include the full
license text as well as the list of authors. This is something Padre
does not.
More[1], it fails the Debian Free Software Guidelines[2], therefore
cannot be part of Debian. (There is Debian package named
tango-icon-themes in non-free).
[1] http://evan.prodromou.name/
Debian_Creative_Commons_Workgroup_report
[2] http://www.debian.org/social_contract#guidelines
So, as I see it:
* Padre is not distributable, as GPL and CC-SA-2.5 conflict[3];
* Padre is breaking CC-SA-2.5 by not distributing the full license
text and authors list;
* Debian cannot ship Padre with CC-SA-2.5-licensed material in it.
[3] I am not sure how CC-SA-2.5 interoperates with Artistic
license
Is chosing another icon set an option?