In short, the CSS file (used for styling the content in terms of colors, sizes, positions, backgrounds, etc) is either blocked or being overridden by a local style sheet. The former is the most likely since it is a cross-browser issue, so I'd check your router settings (if applicable), HOSTS file, any security/filtering software, etc. Specifically, look for rules and log file entries that target files ending with the extension CSS and/or the respective websites.

In addition, Chrome and Safari, and Firefox with various extensions, enable you to watch what files are required/downloaded by the webpage. You'll usually find them listed under developer tools, usually with a "resource tracker" reference. Again, look for CSS files among the listed resources.

Note what changes you've made lately regarding security/privacy/filtering software and your HOSTS file, and also see if you can grab a screenshot of the CSS part of the resource list from one of the browsers, posting that here if you don't see an obvious issue.

Let us know.
John