Using an image as background

The block elements containing text on my website use cream coloured text on a dark green background (links in blue-grey). The blocks of text are centrally positioned on a background of vertical narrow dark green and light green stripes (if this sounds horrible, don't worry -- it's an experiment!).

I've changed the colour scheme of PunBB to use the cream, greens and blues of my website, but I'd like to use the stripy background there as well. On my site it's a small jpg which is repeated horizontally and vertically. how would I implement this in PunBB?

In Allsorts_cs.css (the file that's in the style/imports folder) there is a line BODY {BACKGROUND-COLOR: #A5C8A5}. Can I simply use CSS there and change it to BODY {BACKGROUND-IMAGE:URL(images/stripenarrow4.jpg);} ?

Sort-of-connected question (sorry if it's rather a basic one -- I am completely self-taught on HTML, css and PHP): is there a reason for using all caps in the .css files?

Thanks.