fix(challenge modal): snap close button to whole pixels

src/components/challenge-modal/challenge-modal.module.css

@@ -40,6 +40,8 @@
   border: none;
   image-rendering: pixelated;
   background-image: var(--close-button-background-image);
+  background-position: center;
+  background-repeat: no-repeat;
 }
 
 .container input, .container textarea {
@@ -160,4 +162,4 @@
     border-left: none;
     border-right: none;
   }
-}
+}