Before installing, click on the color value (unless you want to keep the default green color) - a color picker will pop up. Choose your color, then install. Note: I suggest you don't pick a very bright color, as the white text from the header would then become unreadable.

If after installing the color doesn't suit you, uninstall, come back to this page and reinstall with a different color (or change the color manually, by editing the css color code in the user style).

This user style will not update automatically, so your customization will be preserved on your computer.

This user style will ONLY change the board background color (the whole page actually). If you want a similar custom color theme, but with a black top bar and additional styles, check out my http://freestyler.ws/style/75464/trello-theme-custom-board-color-with-black-bar