Now battery <= 30 gives a warning color

.config/waybar/style.css

@@ -135,6 +135,10 @@ button:hover {
     background-color: #29ad60;
 }
 
+#battery.warning {
+    background-color: #ffa000;
+}
+
 @keyframes blink {
     to {
         background-color: #ffffff;