Customized World Type Settings: Previous Page Button

Top left (Previous Page) button turn into clickable button in first page when you click on "Defaults" button

Go to Singleplayer > Create New World > More World Options... > World Type: Customized > Customize

Now, click on "Randomize" then "Defaults", warning pop-up will appear, if you click on "yes" or "no" button, "Previous Page" button will be clickable