Use bootstrap backgrounds and text utilities for read issue reports

app/assets/stylesheets/common.scss

@@ -2297,8 +2297,3 @@ input.richtext_title[type="text"] {
     display: none;
   }
 }
-
-.read-reports {
-  background: $lightgrey;
-  opacity: 0.7;
-}

app/views/issues/show.html.erb

@@ -24,14 +24,14 @@
     <h3><%= t ".reports_of_this_issue" %></h3>
 
     <% if @read_reports.present? %>
-    <div class="read-reports">
+    <div class="bg-light text-muted">
       <h4><%= t ".read_reports" %></h4>
       <%= render "reports", :reports => @read_reports %>
     </div>
     <% end %>
 
     <% if @unread_reports.any? %>
-    <div class="unread-reports">
+    <div>
       <h4><%= t ".new_reports" %></h4>
       <%= render "reports", :reports => @unread_reports %>
     </div>