Make the dropdown arrow a CSS variable so it can be customized.
Make the dropdown arrow a CSS variable so it can be customized.

In different themes, having just a black arrow can look bad. This exposes it as a variable, which is our public interface for customization.

Fixes #1159.
ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets