default constructor for theme()

[moodymudskipper]

By default we guess the theme, but this means if it is guessed (e.g. theme_bw())we cannot see how the theme() call would look like. and the list() call is less readable because all the default values are enumerated.

We need a "default" constructor that behaves as "theme" does now, and a "theme" constructor that only uses theme()