How to disable smart cast highlight Kotlin?
go to File -> Settings -> Editor -> Color Scheme -> Java/Kotlin -> Classes and Interfaces
You should be able to find color scheme for classes here. Hope it helps.
In the IDE prefererences, see:
Editor → Color scheme → Kotlin → Smart-casts → Smart-cast value
If you want to disable value smart-cast highlighting, just uncheck Background.
You may want to configure the other kinds of smart-casts, too.