Changing the Skeleton theme slightly

Sure. You have to change some CSS rules in your derived skin (please derive the skeleton skin before tweaking it to preserve your settings):

• for the grey sides, decrease body padding from 3% to whatever you want (zero is acceptedand won't need a % sign)
  
• for the size of the side panel, you will have to change two rules : increase both the width of main_panel from 72% and margin-right of the side_panel from 75% by a few points

It is really easy to find out what to change once you get used to checking the layout with the Firebug extension for Firefox. Check it out, it will be one of the best investment of your time in your new web design carreer, I swear!