Folder Panel Background Color

There is no theme for this space in the theme editor, so we can't change the color of only this space via the theme editor. This space does use Color > Secondary > Light, so you could change this color to be the color you want the side area to be. However anything else that uses Color > Secondary > Light, will also use this new color you chose.  

If you are open to using CSS to change the color, the attribute is .dp-portal-sidebar-header. Once you upload the CSS, you will need to set it as the Default CSS to see it reflected in the environment.

.dp-portal-sidebar-header{
    background: yellow
}

https://documentation.decisions.com/v8/docs/uploading-css