@bkleinhe: Thanks for your patience.
Please remove the following css from your custom css box:
#secondary {
width: 25.5%;
}
.left-sidebar #content {
margin: 0 3% 0 28.6%;
}
#content {
width: 68.4%;
}
@media screen and (min-width: 1100px) {
    .left-sidebar #secondary {
        float: left;
        margin-left: 1.4%;
        margin-right: 0;
    }
    
    .hentry {
       padding:0;
    }
    
    #page {
        width:1040px;
    }
}
.left-sidebar #primary {
    float: right;
    margin: 0 0 0 -35.5%;
}
#primary {
    float: left;
    margin: 0 -35.5% 0 0;
    width: 100%;
}
Now to change the width of Content and Sidebar, you can just change the width of primary and secondary. Like following css:
@media screen and (min-width: 961px) {
    #primary { width: 73%; }
    #secondary { width: 25%; }
}