Hi Martin,
Your skin looks like its coming along well. It has a nice clean feel. I share your pain on the icon situation, I had to make all new icons for the Atlantis and the new SmarterMail 5.x skin which, yes, can take some time. We are looking to have a transparent icon file available for SmarterMail 5.x to aid in custom skinning. In your situation I had two suggestions.
The first, if you are quite proficient in Photoshop, you could cut out all the background pixels in each one of the icons. Then to eliminate the harsh edges, use the blur tool sparingly. This will be a little tricky and time consuming.
My second suggestion is to either create or purchase your own icon set specifically for this skin. You can find icon sets that are decent both in quality and price. Have you ever visited templatemonster.com? They have a large selection of icon sets and chances are you could proabably find one to match your orange color. Last time I checked, templatemonster.com was selling icon sets for under $100.
One other quick note, I know your skin is not finished so this is more of a reminder, any custom skin must still have "SmarterMail [Edition] [Version #]" visible with a minimum of 8pt font size. It can say, "Powered by SmarterMail [Edition] [Version #]." This requirement can be found in the EULA.