Question about the colors in the plugin development

Hello.

I'm trying to develop a basic IntelliJ plugin.

So I created a “ToolWindow” from the “ToolWindowFactory” and added a “JPanel” to it.

My question : How can I change the JPanel background ?

I tried to change with “setBackground”, “setVisibility” and “setOpaque” function but nothing changes.

Can you help me ?
 

Bet regards.