Hi, do you want to increase the sidebar width or decrease?

You can do this with changing columns classes with jquery;

try adding the code below to the External JavaScript/CSS box under Theme Settings > Style tab.

<script>
jQuery(document).ready(function($) {
    $('.content.span8').removeClass('span8').addClass('span9');
    $('#sidebar.span4').removeClass('span4').addClass('span3');
    $('.post-list .post .media-box + .span5').removeClass('span5').addClass('span6');
    $('.post-list .post > .span8').removeClass('span8').addClass('span9');
});
</script>

since we are using responsive grid in bootstrap, either you will use 4 column for sidebar (default) or 3 column (as i described above), there is no 3.5 column in this grid system; hahaha :)

Thanks.