1. Open Theme Editor under Appearance Menu.
2. In the Style.css file, try to find this:
#secondary,
#tertiary,
.left-sidebar #secondary,
.left-sidebar #tertiary{
width: 26.7%;
max-width: 280px;
margin-right: 0px;
padding-right: 10px;
}
The above code is for your side bar. First reduce max-width:280px
to some width that you want your sidebar to have, for example: change to 150px. Then, try to find this code again:
#content, .left-sidebar #content {
width: 69.6%;
max-width: 730px;
}
This code is for your main content. And now in this code, try to change max-width such that the total of content’s max-width and sidebar’s max-width equals 1010x. In our example, this will change to: 1010-150=860px.
And finally, play around with the width percentage for content so as to suit to your taste.
Use firebug if you are using firefox or InspectElement if you are using Chrome. This makes it easier to play around with css.
Hope you’ll find this useful.