Update style of "nominal-dynamic" bar to match style of "nominal" bar

[keller-mark]

An optional suggestion:

How about making this track visually similar to nominal type tracks and showing user-annotated categories (e.g., "Positive") as text labels? Initially, the bar can be gray colored and have a placeholder text (e.g., "Undefined"), and whenever a user clicks on a bar, the color and the text changes accordingly.

Originally posted by @sehilyi in #285 (comment)