Increase width of content

Hi,

I’m trying to increase the width of my content area on a twenty eleven theme, and have looked at loads of threads, but nothing seems to work – I have added the following to my child styles.css:

/*Increase width of content*/
#content {
margin: 0 7.5% 0 7.6%;
width: 85%;
}

but, it doesn’t work – url – https://surfacedeals.co.uk/ – I thought it might be the wp-ui plugin I’m using, but I notice that the

nav that I added at the top of the page has not increased in width either (so, I don’t think wp-ui plugin is the problem)- any help would be much appreciated. (I’m trying to get the content width to almost match the header width)

Also, I really love the way people help each other out on the forum, but is there some documentation that explains what each style in the default .css file does? Or at the very least describes each area on a WP page and it’s parameters? (If there is I may eventually graduate from being a perennial ‘newbie poster’ and actually graduate to helping other people out myself once I’ve understood what all the code means…)

Thanks Earl