refactor(styles): simplify color variables and update CSS constants

Add new color palette file and refactor constant definitions to use variable tokens.

src/styles/notification/notification_close.css

@@ -0,0 +1,24 @@
+#notif-close-button {
+  background-color: var(--surface);
+  border-radius: apply(radius-lg);
+  padding: 4px;
+  transition: background-color apply(transition-fast);
+}
+
+#notif-close-button:hover,
+#notif-close-button:focus {
+  background-color: var(--surface-bright);
+}
+
+#notif-close-button:active {
+  background-color: var(--red-dim);
+}
+
+#notif-close-label {
+  color: var(--red-dim);
+  font-size: 20px;
+}
+
+#notif-close-button:active #notif-close-label {
+  color: var(--shadow);
+}