How to change the background color from the default white to black

Hello Moin,

This is what i have. It doesn’t have a button for me to apply the color scheme. It gives me css code, but i don’t know where can i apply the css code. I cannot find ‘customize’ as what NateDhaliwal suggested.

If i click ‘copy to clipboard’, and i paste it in a notepad. This is what i see.