OK. I've had a quick look and I think that your immediate problem is nothing to do with your CSS.
If you look at the code extracted from the HTML that Zen Cart is generating for you, you will see that your left hand column sits within a table cell with a fixed width of 150px. The contents of the column then sit inside a div with a fixed width of 138px and each box inside that also has a pre-defined width of 138px.
PHP Code:
<td id="navColumnOne" class="columnLeft" style="width: 150px">
<div id="navColumnOneWrapper" style="width: 138px">
<!--// bof: categories //-->
<table width="138px" border="0" cellspacing="0" cellpadding="0" class="leftbox" id="categories-table">
As a result you have boxed of 138px centered in a column of 150px giving you a 6px margin each side. I suspect that you gone into the layout settings page under the Configuration menu in Admin and reduced the Column Width - Left Boxes value at the top but left the Column Width - Left value about 14 lines down, unchanged. Make this 138 too and you shoudl solve your immediate problem. But it will just be the first of many.
You are using the classic template. I explain here why that's not ideal and link to a better approach.
However, you are setting yourself up for even more grief than most people because you have added the stylesheet.css to the stylesheet_new.css and the stylesheet_original.css already included in the classic template package. This means you have three stylesheets, with overlapping functionality, competing for your user's browser's attention, and the one that you are editing is called first and so will be the overridden by the others if there's a comflict. I strongly recommend switching to your own template based on the template default before you get in too deep.