refactor: rename style file

.vitepress/theme/style/custom.css

@@ -1,4 +1,4 @@
-/* var.css */
+/* custom.css */
 :root {
   --vp-c-brand-1: #18794e;
   --vp-c-brand-2: #299764;

.vitepress/theme/style/index.css

@@ -1,2 +1,2 @@
 /* index.css */
-@import "./var.css";
+@import "./custom.css";