In your browser, right click Inspect and see the css of your accordion. You will see this is the css that allows you to change it:

Hello @Rafael Moreira again,

You can use the class "background-green" to add background color green or "text-green" to add the text green.

Hope it helps,

NunoR

Hello Rafael Moreira,

if you want to change the background color for title side

to get above result add this css in your application 

.section-expandable .section-expandable-title {

    background-color: green;

}

To change background color of the expendable area of accordion item as shown below Image

.section-expandable .section-expandable-content-expanded, .section-expandable .section-expandable-content.is--expanded {
    background-color: green;
   
}

Used this CSS to your application screen level 

Thanks and Regards,

Akshay Deshpande