FrameStyle is an option for two-dimensional graphics functions that specifies how the edges of a frame should be rendered.
FrameStyle -> style specifies that all edges of the frame are to be generated with the specified graphics directive, or list of graphics directives.
FrameStyle -> xmstyle, ymstyle, ... specifies that different edges of the frame should be generated with different styles. The edges are ordered clockwise starting from the bottom edge. All styles must be enclosed in lists, perhaps of length one.
Styles can be specified using graphics directives such as Dashing, Hue and Thickness.
The default color of frame edges is specified by the option DefaultColor.