Fix hr elements

Remove the `color` attribute (`background-color` is the correct way).
diff --git a/static/css/style.less b/static/css/style.less
index 7a0c99c..e1be931 100644
--- a/static/css/style.less
+++ b/static/css/style.less
@@ -47,7 +47,6 @@
 }
 
 hr {
-  color: @border_color;
   background-color: @border_color;
   height: 1px;
   border: none;