refactor: Extract card style to theme file

src/theme.js

@@ -0,0 +1,21 @@
+import { Platform } from "react-native";
+
+const card = {
+    borderColor: '#e1e8ee',
+    borderWidth: 1,
+    ...Platform.select({
+        ios: {
+            shadowColor: 'rgba(0,0,0, .2)',
+            shadowOffset: {height: 0, width: 0},
+            shadowOpacity: 1,
+            shadowRadius: 1,
+        },
+        android: {
+            elevation: 1,
+        },
+    }),
+    padding: 8,
+    margin: 8,
+    backgroundColor: '#fff',
+};
+export default {card};