Create initial theme for RadGrad

[philipmjohnson]

Define a set of CSS classes and LESS variables to implement the initial theme.

• All variables and CSS classes should start with "radgrad-".
• Keep the color names out of the variable and class names so that the colors can change over time without impacting on the variable and class names.
• Provide CSS classes for all elements that need adjustment so that they can be tweaked individually in the theme later on with as little impact as possible on the RadGrad code base.