I just sign up and explore the website, the dark theme set by default but I can't find the button to change to light.
I explored the website as well but I didn't have to register myself but as I see, I like the dark theme that they have by default. And you can adjust it into light by looking at the left side bottom side under help which has a label of "AM-PM".
You can press the sliding button to change the theme at your will.