Show current tab even in high contrast mode

In the dashboard header and the course navigation bar, the current
tab is supposed to be the only one with a bottom border.  But in
high contrast mode in Windows, all of them were assigned a bottom
border color.

So instead of toggling the color when active, toggle the border
style, since Windows overrides our color choice anyway.

LEARNER-3219

lms/static/sass/_header.scss

@@ -102,12 +102,12 @@
             font-weight: $font-weight-normal;
             display: inline-block;
             margin-bottom: -1*$baseline/2;
-            border-bottom: 4px solid transparent;
+            border-bottom: 4px hidden theme-color("dark");
             cursor: pointer;
 
             &.active,
             &:hover {
-              border-bottom-color: theme-color("dark");
+              border-bottom-style: solid;
             }
 
             &:hover {

lms/static/sass/course/layout/_courseware_header.scss

@@ -52,13 +52,13 @@
         display: block;
         text-align: center;
         text-decoration: none;
-        border-bottom: 4px solid transparent;
+        border-bottom: 4px hidden theme-color("primary");
 
         &:hover,
         &:focus,
         &.active {
           color: theme-color("primary");
-          border-bottom-color: theme-color("primary");
+          border-bottom-style: solid;
           background-color: transparent;
         }
       }