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 the mentioned app they are made for. If you want to distribute them then give credits to me and please link to this blog post.