[Foreground] Increase page text width to 90% of available space, but how?

I'm just evaluating foreground for a new site that I'm developing, but I'm finding that the text in the middle of the screen is too narrow. I have text wrapping round the TOC's of my articles, and with the article text beside it, the whole page looks out of proportion. So how can I increase the text width to take up about 90% of the page width please? Thanks Sharon. -- A taste of linux = http://www.sharons.org.uk TGmeds = http://www.tgmeds.org.uk DrugFacts = https://www.drugfacts.org.uk Debian 8.6, fluxbox 1.3.5-2, emacs 25.1.1.1