Hi there, if your skin is not Artisteer-based, it definitely shouldn't have ".art-post" selectors in it. I changed the mojoPortal demo site to the andreasviklund-02-alt1 skin and examined the css with Firebug. Div class .modulecontent has padding of 0 4px 4px 7px (top, right, left, bottom), so chances are that is what you will want to change in your skin's css class to fine tune that module spacing.
Don't forget to turn off CSS and style sheet combining during debugging so you don't drive yourself crazy. You can do that with these directives in your user.config:
<add key="CombineCSS" value="false" />
<add key="MinifyCSS" value="false" />
<add key="CacheCssOnServer" value="false"/>
<add key="CacheCssInBrowser" value="false"/>
Be sure to turn those back on when you're done with skin development. If you're running a very recent version of mojoPortal, you can make use of the Disable CSS Caching button in Administration, Advanced Tools, Design Tools.
Jamie