New themes suitable for extending

I added some new themes which are not meant to be used directly but are designed to make extending and writing new themes easier.  I noticed when writing themes that using `theme_gray()`or `theme_bw()` as the base theme required finding which non-top level elements were defined by those themes.   `theme_base0` is a  complete theme that only defines top-level elements.  `theme_base_sizes` defines various size elements that are used in `theme_gray`, and users are less likely to want to alter than colors.  `theme_base` combines `theme_base0` and `theme_base_sizes`.