Fix cancel link color for when the header has dark background

[genadis]

when dark color is used for header for example: shepherdHeaderBackground: 'rgb(20,20,20)' the cancel link color is broken:

• when there is no title, the 'x' cancel link is white and not visible at all.
• when there is title the 'x' cancel link is white and there is no transparency at all and no hover effect.

This fix, fixes it without breaking light header backgrounds.