For inline stylesheets, you can get the content out of the normal DOM like with any other element:
document.getElementsByTagName('style')[0].firstChild.data
For external, linked stylesheets it's more problematic. In modern browsers, you can get the text of every rule (including inline, linked and @imported stylesheets) from the document.styleSheets[].cssRules[].cssText property.
Unfortunately IE does not implement this DOM Level 2 Style/CSS standard, instead using its own subtly different version of the StyleSheet and CSSRule interfaces. So you need some sniff-and-branch code to recreate rules in IE, and the text might not be exactly the same as the original. (In particular, IE will ALL-CAPS your property names and lose whitespace.)
var css= [];
for (var sheeti= 0; sheeti<document.styleSheets.length; sheeti++) {
var sheet= document.styleSheets[sheeti];
var rules= ('cssRules' in sheet)? sheet.cssRules : sheet.rules;
for (var rulei= 0; rulei<rules.length; rulei++) {
var rule= rules[rulei];
if ('cssText' in rule)
css.push(rule.cssText);
else
css.push(rule.selectorText+' {\n'+rule.style.cssText+'\n}\n');
}
}
return css.join('\n');