Styles consists of two things, the CSS which lives in /styles/ and the templates which live in templates/styles/
now simple and tranquil you will already have in your Tiki install unless you deleted them. In which can you can just download the tar or zip again and copy the CSS file from styles/ into your tiki sites /styles/
There are some premium themes available at http://tikihost.net.
Those come with CSS and templates and just need copying into the two folders /styles/ and /templates/styles/ and its installed.