So, Nokia Belle OS is coming soon. I am already using a custom firmware for Belle by “The One” in my Nokia N8.
Previously, defining a custom icon for an app needed to be done via Symbian theme designer. You could also use SisContent to manually apply icons. You would need the App UID and a SVG icon for the app. This was cumbersome.
I found a nifty little app called Icon Changer from LanternSoft who make some awesome utility apps for Symbian. What you need is just a PNG file which is easier to create compared to a vector file which needs more learning for creating properly optimized SVG files via Illustrator or InkScape.
I have made some icons for my personal use which I am releasing here. You can use them for for the mentioned app they are made for. Do not re-distribute them without my permission. You can link to this blog for downloading.