In your terminal, test the themes by setting the following environment variables; basically, just copy and paste it and then open nnn.
After you decided one permanently, just set it in your rc shell, for example, append the line in ~/.bashrc for bash or ~/.zshrc for zsh.
White & blueish pastelized.
export NNN_COLORS='#7b9f7b9f;1234' NNN_FCOLORS='c1e2ff6ff36dccf7c6e3aba1'
Golden pastel. by @Schwarztee
export NNN_COLORS='#a744b322' NNN_FCOLORS='dfdfdf6c0000df42bbbaba5f'
As you can see, we have really only a few themes, you can help by making your own and opening a pull request, I'll be happy to add it on the list. A little bit to help, use the official n³ configuration to set colors of your theme and test with this official n³ script.