18
There are some hacks you can use in the CSS to target only Safari, such as putting a hash/pound (#) after the semi-colon, which causes Safari to ignore it. For example
.blah { color: #fff; }
.blah { color: #000;# }
In Safari the colour will be white, in everything else it will be black.
However, you shouldn't use hacks, as it could cause problems with browsers in the future, and it may have undesired effects in older browsers. The safest way is to either use a server side language (such as PHP) which detects the browser and then serves up a different CSS file depending upon the browser the user is using, or you can use JavaScript to do the same, and switch to a different CSS file.
The server-side language is the better option here, as not everyone has JavaScript enabled in their browser, which means they wouldn't see the correct style. Also JavaScript adds an overhead to the amount of information which needs to load before the page is properly displayed.
Safari uses WebKit, which is very good with rendering CSS. I've never come across anything which doesn't work in Safari, but does in other modern browsers (not counting IE, which has it's own issues all together). I would suggest making sure your CSS is standards compliant, as the issue may lie in the CSS, and not in Safari.