Removed ugly gray canvas color.

Setting the facecolor of the figure to 'none' removes the gray, setting
the background color to whatever the window's background color is (by
making it transparent).
Set the chart title to use the system theme's color.
Used the system's color theme to color the chart figure and title.

Undid setting the figure to be transparent.
Used wx.SystemSettings to make the window look 'native' in all themes.

Transparency worked, but caused the figure's title to be drawn over
itself each call of canvas.blit(), each time the user moused over a
wedge.