Selected Conversations No Longer Change Colour

fixes CNVS-17759

Brings back the blue highlight for active conversations. Uses our
background color variable to meet accessibility with high-contrast.

Test plan:
- Go to conversations
- Highlight a conversation and now it should have a blue background
to let you know it's the active message

Change-Id: Icbbbc37094667943ecfa3a4ca82d44f2355db6ae
Reviewed-on: https://gerrit.instructure.com/47985
Reviewed-by: Chris Hart <chart@instructure.com>
Product-Review: Chris Hart <chart@instructure.com>
Tested-by: Jenkins
QA-Review: Steven Shepherd <sshepherd@instructure.com>

Author: Colleen Palmer

app/stylesheets/pages/conversations/_conversations_new.scss

@@ -182,9 +182,11 @@
     position: relative;
 
     &.active {
-      background: lighten($canvas-neutral, 10);
+      background: $can-bg-light-primary;
+    }
+    &:hover {
+      background: $can-bg-light-primary;
     }
-
     &:hover .message-actions a, .message-actions a:focus {
       left: 0;
       outline: none;