If you're running the latest V7 FULL of my Classic WB, then you've all the icon programs you'll need.
Under the icons menu is an advanced Icon editor, an update program supporting drag n drop and new/normal icons, a killnewicon script for removing newicons and finally a neat program called picticon. Picticon transforms pictures (jpeg/iff/bitmap etc) into icons! Great for games.
As for icon extra's to add a different look to the workbench, just use good old Aminet
. Search through the archives and you'll find icons galore! Download an icon set and use the aforesaid programs to update. You can use normal icons or newicon sets, all are supported in the Classic WB. You might have to increase the screen mode colours from 8, use interlaced screen-modes/picasso96 in order for them to look good however.
I'm not sure about good theme sites. Perhaps someone else can help.