To change the default background colour for your game, go to Options > General and click on the Background colour swatch.
To change the background colour whilst the game is being played, you need to use the bgcolor tag. The format is
<bgcolor="colour">
where colour can be a named colour (red, green, blue etc) or an RGB Hex colour such as #FE6E59. You can revert to the default background with the named colour "default".
You can place this anywhere within your output text, allowing you to change the colour at any point in your game.