CartoCSS allows you to create complex map styles with a CSS like syntax. The CSS syntax and features like variables and attachments help you to manage complex styles: A few hundred lines of CartoCSS can result in a few thousand lines of MapServer stylings.
Magnacarto is a command line tool and we have binaries for Windows, Linux and Mac OS. Detailed informations and a download link for the binareis you will find on Github.
We are also working on a web-frontend for magnacarto that shows you an interactive map with the final styling. It supports multiple views of your map and reloads of the maps automatically as soon as you make changes to the styling.