Set a white background-color on checkboxes

Otherwise they appear with the same color as the background, which is
not what we want. They should always be white.

app/styles/input.css

@@ -90,6 +90,7 @@ option {
 input[type=checkbox] {
   position: relative;
   appearance: none;
+  background-color: white;
   border: 1px solid dimgrey;
   border-radius: 3px;
   width: 0.8em;