Simplify border and background in admin header

Instead of defining them for all headers and then overwriting them, we
can add it just to the body of the public layout.

app/assets/stylesheets/admin.scss

@@ -52,13 +52,9 @@ $table-header:   #ecf1f6;
     float: none;
   }
 
-  .header {
-    background: transparent;
-    border: 0;
-  }
-
   .top-links {
     background: #000;
+    color: $white;
 
     a {
       line-height: rem-calc($line-height * 1.5);
@@ -67,7 +63,6 @@ $table-header:   #ecf1f6;
 
   .top-bar {
     border-bottom: 1px solid #eee;
-    color: $text;
     height: auto;
     padding-top: $line-height / 2;
 

app/assets/stylesheets/layout.scss

@@ -473,8 +473,6 @@ a {
 
 body > header,
 .wrapper > header {
-  @extend %brand-background;
-  border-bottom: 1px solid $border;
   margin-bottom: $line-height;
 
   @include breakpoint(small down) {
@@ -517,6 +515,12 @@ body > header,
   }
 }
 
+.public > .wrapper > header,
+.proposal-dashboard > header {
+  @extend %brand-background;
+  border-bottom: 1px solid $border;
+}
+
 .top-bar {
   background-color: inherit;
   padding-bottom: 0;