Apply @color argument to striped gradient

The @color argument is currently not being used for the striped gradient.
Remove progress bar striped @color parameters; use default transparent white.

* If the @color parameter is not removed, the stripes are the same color as the element background color. In this case, the stripes do not appear.
Merge pull request #9968 from thelukemcdonald/master

Apply gradient stripe @color parameter