How do I widen my text margins using Twenty fourteen template?

I need my text margins to be wider. There is no option to widen them…so much wasted space. The photo gallery page would look so much better with at least 2 columns of pictures, not a single column. The site I’m working on is https://www.perpetualbloomgardening.com/test. Also, is it possible to get rid of the black background, so that it’s just the custom background picture visible?? My knowledge of web editing is very limited, but any help is much appreciated.