Changing the Skeleton theme slightly

Can you point the way?

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I would like to change the proportions of the inset (there is a grey outsude area that I would like to make smaller) and left column (which I would like to make narrower) in the skeleton theme.

See the attached screen shot

Any idea which parts of the theme I would have to adjust.

Chang colors and fonts through the CSS files is no problem neither changing the various images.