--- a/calendar/base/themes/windows/calendar-unifinder.css
+++ b/calendar/base/themes/windows/calendar-unifinder.css
@@ -75,8 +75,23 @@
   }
 
   #unifinder-search-field:not(:-moz-lwtheme)[focused],
   #unifinder-task-edit-field:not(:-moz-lwtheme)[focused] {
     background-color: hsla(0, 0%, 100%, 1);
     border-color: #4595e5;
   }
 }
+
+@media (-moz-os-version: windows-win8),
+       (-moz-os-version: windows-win10) {
+  .unifinder-closebutton {
+    -moz-image-region: rect(0 20px 20px 0);
+  }
+
+  .unifinder-closebutton:hover {
+    -moz-image-region: rect(0 40px 20px 20px);
+  }
+
+  .unifinder-closebutton:hover:active {
+    -moz-image-region: rect(0 60px 20px 40px);
+  }
+}

--- a/calendar/base/themes/windows/today-pane.css
+++ b/calendar/base/themes/windows/today-pane.css
@@ -88,8 +88,25 @@
       background-clip: padding-box;
     }
   }
 
   #today-pane-panel .folderview-cycler {
     margin-top: -1px;
   }
 }
+
+@media (-moz-os-version: windows-win8),
+       (-moz-os-version: windows-win10) {
+  .today-closebutton {
+    padding-top: 0;
+    padding-bottom: 0;
+    -moz-image-region: rect(0 20px 20px 0);
+  }
+
+  .today-closebutton:hover {
+    -moz-image-region: rect(0 40px 20px 20px);
+  }
+
+  .today-closebutton:hover:active {
+    -moz-image-region: rect(0 60px 20px 40px);
+  }
+}