Go to Settings/Triberr in your Wordpress dashboard. See image.
In the Triberr plugin configuration, enter the desired hex code for the comment box area.
You may be wondering how you might know what the hex value of your blog background might be?
I use Eye Dropper. It's for Chrome browser.
If you're using a different browser, do a google search to find the right method for your browser/operating system.
And if you're still having a hard time figuring it out, tweet me and I'll help you @dinodogan