Changed the colour of the sliders to match colour scheme. The
knob is the same colour as the toolbar, and the slider itself
is the same colour as the buttons on the toolbar.

See teal:
http://www.google.com/design/spec/style/color.html#color-color-palette

BUG=https://github.com/jackhou-chromium/bitmapper/issues/22
R=tapted@chromium.org

Review URL: https://codereview.appspot.com/187580043