Separate reading and hover states a bit

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

@@ -229,11 +229,18 @@
   padding: ($baseline/4) ($baseline/2);
   transition: none;
 
-  &.is-active,
   &:hover {
     color: $forum-color-background;
     background-color: $forum-color-hover-thread;
+  }
 
+  &.is-active {
+    color: $forum-color-background;
+    background-color: $forum-color-reading-thread;
+  }
+
+  &.is-active,
+  &:hover {
     .forum-nav-thread-labels > li {
       border-color: $forum-color-background;
       color: $forum-color-background;

lms/static/sass/discussion/utilities/_variables-v1.scss

@@ -15,6 +15,7 @@ $forum-color-marked-answer: $green-d1 !default;
 $forum-color-border: $gray-l3 !default;
 $forum-color-error: $red !default;
 $forum-color-hover-thread: $gray-d3 !default;
+$forum-color-reading-thread: $gray-d3 !default;
 
 // post images
 $post-image-dimension: ($baseline*3) !default;  // image size  + margin

lms/static/sass/discussion/utilities/_variables-v2.scss

@@ -15,6 +15,7 @@ $forum-color-marked-answer: $green-d1 !default;
 $forum-color-border: palette(grayscale, base) !default;
 $forum-color-error: palette(error, accent) !default;
 $forum-color-hover-thread: palette(grayscale, dark) !default;
+$forum-color-reading-thread: palette(grayscale, dark) !default;
 
 // post images
 $post-image-dimension: ($baseline*3) !default;  // image size  + margin