LMS: Adding missing third topic state

lms/static/sass/discussion/elements/_navigation.scss

@@ -176,13 +176,15 @@
   background-color: $gray-l6;
 
   &.is-unread {
+    background: $white;
+    color: $blue-d1;
 
     .forum-nav-thread-comments-count {
-      background-color: $blue-d1;
-      color: $white;
+      background-color: tint($blue-d1, 90%);;
+      color: $blue-d1;
 
       &:after {
-        border-right-color: $blue-d1;
+        border-right-color: tint($blue-d1, 90%);;
       }
     }
   }
@@ -192,13 +194,8 @@
   display: block;
   padding: ($baseline/4) ($baseline/2);
 
-  &.is-active,
-  &:hover,
-  &:focus {
-    background-color: $forum-color-active-thread;
-  }
-
   &.is-active {
+    background-color: $white;
     color: $base-font-color;
 
     .forum-nav-thread-comments-count {
@@ -210,6 +207,19 @@
       }
     }
   }
+
+  .forum-nav-thread-comments-count {
+
+    // topic read, but has unread comments
+    &.is-unread {
+      background-color: $blue-d1;
+      color: $white;
+
+      &:after {
+        border-right-color: $blue-d1;
+      }
+    }
+  }
 }
 
 %forum-nav-thread-wrapper {