HTML styles: fix style of hr so it works when printed (#10535)

Previously `background-color` was used to style the hr, but this gets ignored
when printing.  This commit uses `border-top` instead.

Author: nathanlesage

data/templates/styles.html

@@ -130,8 +130,8 @@
  overflow: visible;
 }
 hr {
-  background-color: #1a1a1a;
   border: none;
+  border-top: 1px solid #1a1a1a;
   height: 1px;
   margin: 1em 0;
 }

test/writer.html4

@@ -110,8 +110,8 @@
      overflow: visible;
     }
     hr {
-      background-color: #1a1a1a;
       border: none;
+      border-top: 1px solid #1a1a1a;
       height: 1px;
       margin: 1em 0;
     }

test/writer.html5

@@ -110,8 +110,8 @@
      overflow: visible;
     }
     hr {
-      background-color: #1a1a1a;
       border: none;
+      border-top: 1px solid #1a1a1a;
       height: 1px;
       margin: 1em 0;
     }