Use GWT ClientBundle to inject CSS rules

From the GWT docs
(http://www.gwtproject.org/doc/latest/DevGuideUiCss.html#cssfiles),
this is the preferred way to add custom CSS. Using a <link> in WMT.html
is deprecated.
Replace CSS colors with variables parsed by CssResource

Replace hard-coded RGB values with variables. This is excellent! I left
named CSS colors -- e.g., "white" -- unchanged.
Define CSS variable for font family
Define CSS variables for font sizes

Small, medium and large.
Adjust height of TextBoxen to match DroplistBox
Simplify CSS file by combining rules