Style variables for switch control component missing

[NachoJusticia]

Environment

• Package version(s): 3.0.0-beta.1
• Browser and OS versions: Google Chrome, MacOS High Sierra 10.13.3

Steps to reproduce

1. Try to update switch component background colors just for dark theme using dark-switch-background-color variable in _controls.scss file.

Actual behavior

dark-switch-background-color variable is not being used among _controls.scss file, so nothing gets updated.

Expected behavior

• Switch component updates its color just in dark theme.

Possible solution

Update this code (lane 378 of _controls.scss):

.#{$ns}-dark & {
      @include indicator-colors(
        "",
        $switch-background-color,
        $switch-background-color-hover,
        $switch-background-color-active,
        $switch-background-color-disabled
      );
      @include indicator-colors(
        ":checked",
        $switch-checked-background-color,
        $switch-checked-background-color-hover,
        $switch-checked-background-color-active,
        $switch-checked-background-color-disabled
      );
      ...

to

    .#{$ns}-dark & {
      @include indicator-colors(
        "",
        $dark-switch-background-color,
        $dark-switch-background-color-hover,
        $dark-switch-background-color-active,
        $dark-switch-background-color-disabled
      );
      @include indicator-colors(
        ":checked",
        $dark-switch-checked-background-color,
        $dark-switch-checked-background-color-hover,
        $dark-switch-checked-background-color-active,
        $dark-switch-checked-background-color-disabled
      );
      ... 

as I suggest in this PR.