Add support for alternate color themes

[mray]

ZenPacks can override the UI, but there's no support for selecting. A zenoss-user forum post pointed out a practical use for color theming:

"It would be a nice option to have a color scheme that was selectiable on a per user basis. In our main monitoring room the Zenoss page is projected to a whiteboard, and the coloring is really bad off of the projector. I could change the colors in the source but I really do not want to look at the new scheme every time I log in at my workstation."

Ideally we could add a high-contrast theme, a large-font theme and an alternate color theme. Individual users could have select a theme.