There are four themes to choose from to suit your preferences.
The standard theme is the black and blue one known as "Black Water".
The second theme is a blue and silver one known as "Shiva".
The third theme is a black and green based theme entitled "The Darkness".
The final theme is "silt" and is black and green.
They are all dark-based themes, for the simple fact that the white font that is often used here will not be visible to anyone using a light theme.
To change the theme, simply scroll to the bottom of the page, where you should see a heading "Choose a theme:" and then a heading with "Black Water" with up and down arrows. Click on the box that says "Black Water" and choose the desired theme.
This box is depicted below:
I like the third one!! :) :) :) :) :)