@kpax: To increase the width of the content and decrease the width of the sidebar. We need to consider all the responsive design. So, I will give you the default one and then you can adjust the width as per you need. The following CSS can be added in “Appearance => Theme Options => Custom CSS” box.
#primary { width: 754px; }
#secondary { width: 406px; }
@media screen and (max-width: 1224px) {
#primary { width: 654px; }
#secondary { width: 340px; }
}
@media screen and (max-width: 1060px) {
#primary { width: 530px; }
#secondary { width: 340px; }
}
@media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px)
and (orientation : landscape) {
#primary { width: 590px; }
#secondary { width: 340px; }
}
@media screen and (max-width: 960px) {
#primary, #secondary { width: 100%; }
}