correct hover background color for threads

Fixes laCour#78

Test Plan:
- Go to /threads/
- Hover over individual messages inside threads
- Verify the background color changes on hover, but still matches the
  theme and all text is readable

scss/helpers/_variables.scss

@@ -5,6 +5,7 @@ $color-red: #bf360c !default;
 $color-green: adjust-hue($color-red, 140) !default;
 
 $color-shade-darkest: #000 !default;
+$color-shade-darker: #222222 !default;
 $color-shade-dark: #363636 !default;
 $color-shade-mid: #424242 !default;
 $color-shade-light: #545454 !default;

scss/modules/flexpane/_threads.scss

@@ -58,3 +58,16 @@
 #convo_tab textarea#msg_text {
   color: $base-font-color;
 }
+
+ts-message.message:hover,
+ts-message.message:active,
+ts-message.active:not(.standalone):not(.multi_delete_mode):not(.highlight):not(.new_reply):not(.show_broadcast_indicator), ts-message.message--focus:not(.standalone):not(.multi_delete_mode):not(.highlight):not(.new_reply):not(.show_broadcast_indicator), ts-message:hover:not(.standalone):not(.multi_delete_mode):not(.highlight):not(.new_reply):not(.show_broadcast_indicator) {
+  background-color: $color-shade-darker;
+}
+
+ts-thread {
+  .collapse_inline_thread_container:hover,
+  .view_all_replies_container:hover {
+    background-color: $color-shade-darker;
+  }
+}