ContourStyle is an option for contour plots that specifies the style in which contour lines should be drawn.
ContourStyle -> style specifies that all contour lines are to be generated with the specified graphics directive, or list of graphics directives.
ContourStyle -> , , ... specifies that successive contour lines should use graphics directives , ... . The styles must be enclosed in lists, perhaps of length one.
The are used cyclically.
Styles can be specified using graphics directives such as Dashing, Hue and Thickness.
See The Mathematica Book: Section 2.9.7.
See also: PlotStyle.