It changes depending on which board style you're using, there's no universal background color. To figure out the exact color code of whichever style you're using, you can always just take a screenshot of the background, and then use a site that allows you to color-pick/eyedrop the color right from the image! (This is the one I usually use)
However - If you're trying to make something appear invisible, I would recommend you just make whatever it is transparent. That way it'll work across all board styles. If you're trying to make text transparent, you can just use [color=transparent ]text[/color] (without the space.) If you want to make an image or gift transparent, there are several free online editors where you can do so! If you use the forum background color as a replacement for making something transparent, it's not gonna look right for people using different board styles.