Update style.css

Add usability for spoiler O_o

public/style/ForkBB/style.css

@@ -174,6 +174,17 @@ body,
   cursor: pointer;
 }
 
+#fork summary::after {
+  content: "(click to open)";
+  font-size: 0.875rem;
+  margin-left: 0.3125rem;
+  opacity: 0.6;
+}
+
+#fork details[open] > summary::after {
+  content: "(click to close)";
+}
+
 #fork select {
   background-color: #D3B58D;
 }