The border style of the top pane of the splitter. This will apply to the top, left and right border of the top pane.
Default value
'outset'.
Valid values
A fixed set of border style – 'dashed', 'dotted', 'double', 'groove', 'inset', 'outset', 'ridge', 'solid', 'window-inset'. Note that 'window-inset' is only supported by Internet Explorer.
