Converted card width to css var

src/styles.css

@@ -1,4 +1,9 @@
 /* You can add global styles to this file, and also import other style files */
 
+:root {
+  --card-width: 420px;
+}
+
+
 html, body { height: 100%; }
 body { margin: 0; font-family: Roboto, "Helvetica Neue", sans-serif; }