Ubuntu – Xubuntu Mac theme

You cannot do this. Most of the applications in Ubuntu are not HTML web apps embedded running on a common API, like say they would be on WebOS or the new FirefoxOS. However, GTK+ 3.x and Qt 4 and 5 do use a specialized CSS syntax for creating themes.

There is some documentation available in the GtkCssProvider API documentation. There is also some documentation for Qt in the Qt Style Scheets documentation.

However, even if you create some themes, it will not apply to all applications, and any application can still force its own style, if the author of it so chooses.

I end up creating my own theme after all.

Elementary Freya Theme | XfceLook

Feel free to use it.