1. Go to onMessage > Website > Main Site > Styles.
  2. Locate the style.
  3. Hover your cursor over the Edit button for that style. 
  4. In Chrome and Firefox, the unique number for that style appears in the bottom-left of the window pane. The style number here is 10399.User-added image
  5. Alternatively, if you click Edit on the Style in the URL bar, you will see something like https://schoolname.myschoolapp.com/app/onmessage#style/886/14216. Where the 14216 is the Style number.