In events you will often see the option to unlock a Base Theme, or a cosmetic skin for your base. This does not affect gameplay, but will change the way your base looks when you view it or when another player attacks you.
In order to use it, you first go to your Command Center. That will give you the option to "Change Theme." That will show you all the options, where you can either "Preview" it to see what it would like, or "Apply" it to change the Theme your base is using.