Mark sequence link-color scss variable default.

There's already a global $link-color variable defined in
lms/static/sass/base/_variables.scss. Mark the $link-color
variable in sequence scss file with !default, so that the
global $link-color variable gets used, if defined.

common/lib/xmodule/xmodule/css/sequence/display.scss

 $sequence--border-color: #C8C8C8;
-$link-color: rgb(26, 161, 222);
+$link-color: rgb(26, 161, 222) !default;
 // repeated extends - needed since LMS styling was referenced
 .block-link {
   border-left: 1px solid lighten($sequence--border-color, 10%);