How can I get a centred three column site footer in twenty fourteen?

I’m trying to set up a three column site footer on https://kiidstrust.dyndns.org .

It looks half way passable the way I have it at the moment, in a full-width desktop page, but:
– the three Text widgets I’ve inserted seem to be limited in width to 216px and I can’t see where to change it to be wider
– the ‘#supplementary’ containing div for the site footer contents is offset a lot to the right, not centred, and I can’t see where to change it. All the entries for this id in style.css have only very small padding, and no ‘left’ or ‘margin’ settings that I can see.

I’d like the site footer to be width 100% (or 99%), with three equally spaced columns centred over the width, the left side of the left one aligned under the body of the page, and the right column’s right edge aligned under the right hand body of the page.

I’ve searched the theme’s style.css file for 216px and got no matches. Indeed, there aren’t any matches for the .text_widget class in the original theme’s style.css file at all.

I’ve tried setting the width for both the .textwidget container class, and my own .kt_site_footer_widget class, but it makes no difference – the text widgets are fixed at 216px wide no matter what I set in the child theme style.css file.

Can what I want be done? If so, can anyone point me to the settings to change, please, if the problem is in the css? If not, is there another way to get the site footer to display a single wide Text container that I could code for three columns?

PS. The Text widget width shows as 216px in Chrome developer tools, but 219px in Firefox developer tools.

And I’d like the border above ‘Powered by WordPress’ to go all the way across the page, with those words left-aligned with the body copy above. Again, I can’t see how to change these. It’s to do with the #supplementary div being set to the right, which I can’t find a way to change.