background box color

My website is redarrowsc.com. I am very much a newbie at all of this and am just beginning to develop my site. If you look at my home page or bio page, the box that contains content on each page has a white background. I have done searches on the forum and deduced that I can change the color and opacity of the backgrounds but the explanations on how to do so were way over my head. Can someone explain in very basic terms how to change the color and/or opacity of the background of these boxes? Thank you in advance for the help.