How can I widen the content section?

I do not want such wide margins surrounding the content. For one thing, this interferes with tables I have on two posts because I have to make the columns so narrow (page viewers are supposed to print these tables for their own use).

At the very least, is there some way to make the right side bar narrower?

The basic idea would be like this:

#primary {
    width: 75%;
#secondary {
    width: 21.9%;

but there will be some other tweaks required such as changing featured image size (if you use one) and other tweaks to make it constant for all devices but this will definitely get the job done for most.

I was looking to do the same, that is, make the sidebar smaller and the content larger. I tried adding the CSS as suggested and it looks fine on my desktop computer but when I look at it on a mobile phone the sidebar doesn’t drop to under the content like it should instead it sort of cascades down beside the main content. Something like this Pr

Also I am setting up a shop and on mobile devices the woocommerce content looks like it is floated right, which looks a little odd as it leaves space on the left. Is there a way I can get this to behave as other content? It would look better centered or on the left.

Any help greatly appreciated.