Customizing Sans-serif Racer

How to make it a 2 column theme with a more narrow sidebar?

1. Open theme's index.php or Main Index Template file from the right sidebar in Presentation > Theme Editor. Scroll to the bottom of its content and inside <div id="content-sub"> you will see:
   <div id="content-sub" class="bb-t12 bb-fc">
   <div class="bbin-a1">
     <div class="bb-tbase">
       <?php dynamic_sidebar('sidebar-default'); ?>
     </div>
   
     <div class="bb-tbase">
       <div class="bb-t13 bb-fa"><div class="bbin-c1">
         <?php dynamic_sidebar('sidebar-postnav'); ?>
       </div></div>
       <div class="bb-t16 bb-fc">
         <?php dynamic_sidebar('sidebar-links'); ?>
         <?php dynamic_sidebar('sidebar-postrelated'); ?>
       </div>
     </div>
   
     <div class="bb-tbase">
       <?php dynamic_sidebar('sidebar-bottom'); ?>
     </div>
   
   </div>
   </div>

   [Get Code]
2. bb-t12 class sets the width of content-sub to 12 units (out of 30) which is 12/30 x 100 = 40%. bb-fc simply floats the column to the right. Inside content-sub is an additional <div> tag with a class bbin-a1 which sets the left padding (thus a) of the sidebar.
3. However, the reason why currently there are three columns is this:
   <div class="bb-tbase">
     <div class="bb-t13 bb-fa"><div class="bbin-c1">
       <?php dynamic_sidebar('sidebar-postnav'); ?>
     </div></div>
     <div class="bb-t16 bb-fc">
       <?php dynamic_sidebar('sidebar-links'); ?>
       <?php dynamic_sidebar('sidebar-postrelated'); ?>
     </div>
   </div>

   [Get Code]
   where sidebar-postnav widget is placed inside a 13 unit wide column (bb-t12) which is aligned left (bb-fa), while sidebar-links and sidebar-postrelated are in a second column which is 16 units wide (bb-t16) and floated to the right (bb-fc). Class bb-tbase is simply a column base (float:left; width:100%; clear:both;) in which you place the columns.
4. Notice that 13 + 16 = 29 units which leaves 1 unit spacing between these columns. Also the 13 unit column has an inner <div> which sets the right side padding (bbin-c1, thus c).
5. To remove the two additional columns inside the sidebar, simply put the widget "boxes" one after another:
   <div class="bb-tbase">
     <?php dynamic_sidebar('sidebar-postnav'); ?>
     <?php dynamic_sidebar('sidebar-links'); ?>
     <?php dynamic_sidebar('sidebar-postrelated'); ?>
   </div>

   [Get Code]
6. If you want to make the whole sidebar more narrow, change the sidebar wrapper from
   <div id="content-sub" class="bb-t12 bb-fc">...</div>

   [Get Code]
   to
   <div id="content-sub" class="bb-t8 bb-fc">...</div>

   [Get Code]
7. Don't forget the adjust the width of the main content accordingly, from
   <div id="content-main" class="bb-t18 bb-fa">...</div>

   [Get Code]
   to
   <div id="content-main" class="bb-t22 bb-fa">...</div>

   [Get Code]
8. Remember that the sum of column width inside one base should not exceed 30 units. You can always view the CSS Snippets as a reference.