chore: added theme contribution guidelines in theme preview bot (#797)

* chore: added theme contribution guidelines in theme preview bot

* chore: test

* chore: format theme.js

themes/index.js

@@ -249,20 +249,20 @@ const themes = {
     title_color: "82aaff",
     icon_color: "89ddff",
     text_color: "27e8a7",
-    bg_color: "242938"
+    bg_color: "242938",
   },
   slateorange: {
     title_color: "faa627",
     icon_color: "faa627",
     text_color: "ffffff",
-    bg_color: "36393f"
+    bg_color: "36393f",
   },
   kacho_ga: {
     title_color: "bf4a3f",
     icon_color: "a64833",
     text_color: "d9c8a9",
-    bg_color: "402b23"
-  }
+    bg_color: "402b23",
+  },
 };
 
 module.exports = themes;