For the icons see my signature for a auick and dirty generator. Otherwise, Inkscape is a very well written free software that creates svg. I will edit this post in a few minutes and attach a very good boilerblate icon which you can edit on your pc with inkscape or online with the very good svgedit.
Themes in harmattan are css based, so.an ide like NetBeans should be the best option with the drawback tha harmattan css does not seem to be standard css so you will not benefit much from code completion and visual tools in nb.

EDIT. Here it is. Rename as svg