First check if your css is loaded, your website -> right click and select view page source or just press ctrl+u. There you'll find your
style.css. Check if it is showing this
style.css and the right url. Click on it, see if it is loading ok.
It should look like this:
<link rel="stylesheet" type="text/css" href="http://www.exemple.com/skin/frontend/package/theme/css/style.css" media="all" />
For this problem to happen, There are three reasons I know of:
1.) There might be an issue because of System Cache. Clean the cache from the backend or go to
magentoroot/var/ and delete the contents of the cache and session folder.
2.) There can be issue with the file permission of your css File. In this case just type the following command in terminal,
sudo chmod 777 -R /magentoroot/ (update file permissions after Magento creates these new static files. They are not owned by www-data, so they will give 404.)
3.) Lastly, there could be something wrong with the url in your core_config_data table of your database, which I think you have already checked.
4.) Flush your navigator cache :