You can set the background color theme in the preferences (File > Preferences > Style).
![092110.png 092110.png](https://community.jmp.com/t5/image/serverpage/image-id/51371i1B7994447A594C26/image-size/medium?v=v2&px=400)
If you want to do it using JSL, you can write like this. Please beaware that changing a preference will affect the default behavior of JMP.
/* Light Backgroud | Dark Background | Gray Background */
Preferences( Interactive HTML Color( "Dark Background" ) );
Hope this helps.