Q:

How to change default icons in mule while developing custom connector? 

As I am working with the latest devKit (version 3.9.0).  @Icons option is not available in the same.  So is there any other way to change default icons in mule while developing custom connector? 

A:

Created two folders inside the icons folder i.e theme.light and theme.classic and each should have as per following: 

theme.light: 

connectorname-connector-large.png (pixel: 26x26) 
connectorname-connector-small.png (pixel: 16x16) 

theme.classic: 

connectorname-connector-large.png (pixel: 48x32)
connectorname-connector-small.png (pixel: 24x16)

It worked for me.