in stylesheets there already is a rule
if you take it out all hr:s will become visible, but it might not look too good as cmsms default temlates add some hr:s for accessibility in text browsers.
so you can add this to your stylesheets to only show hr:s inside div with id=content
its basic css first part (div#content hr) means that this rule applies to all hr elements inside div with id=content and the part inside {} is the style to be applied.
hope this helps