Fix inconsistency in the active tab colors

The course header sass source is generating styles that eventually
are in conflict with some of the tabs.To avoid it, new style
properties are added that resolves it.

LEARNER-748

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

@@ -48,6 +48,7 @@
         &.active {
           color: $uxpl-blue-hover-active;
           border-bottom-color: $uxpl-blue-hover-active;
+          background-color: transparent;
         }
       }
     }

lms/static/sass/shared-v2/_components.scss

@@ -30,7 +30,7 @@
             border-style: solid;
             border-width: 0 0 4px 0;
             border-bottom-color: transparent;
-            color: $lms-label-color;
+            color: $lms-inactive-color;
             font-size: 14px;
 
             &:hover,
@@ -40,8 +40,8 @@
             }
 
             &.active {
-                color: $lms-active-color;
-                border-bottom-color: $lms-active-color;
+                color: $uxpl-blue-hover-active;
+                border-bottom-color: $uxpl-blue-hover-active;
                 background-color: transparent;
             }
         }

lms/static/sass/shared-v2/_variables.scss

@@ -26,6 +26,7 @@ $lms-border-color: palette(grayscale, back) !default;
 $lms-label-color: palette(grayscale, black) !default;
 $lms-active-color: palette(primary, base) !default;
 $lms-preview-menu-color: #c8c8c8 !default;
+$lms-inactive-color: rgb(94,94,94) !default;
 $success-color: palette(success, accent) !default;
 $success-color-hover: palette(success, text) !default;