@florian,

It contains three different divs. It's shown on the screenshot below. Everything depends on the HTML structure. For the first div, it generates the color by nth-child(even) and nth-child(odd) selectors. You can simply check it by pressing F12 on the browser and opening Chrome or Firefox Console. 

wpForo also allows customizing its design. You can easily write a CSS role and change the background color as you like. 

Thank you for the explanation!