When setting the background color, use the CSS "background" property.

Previously, when Infinity's "Background Color" theme option was set, the
CSS used the "background-color" property.

This makes logical sense, but when compiling the rest of the theme's CSS,
other background theme options could take precedence, thereby not rendering
the background color.

Therefore, this commit uses the CSS "background" shorthand property to render
the background color as this overrides various CSS background properties.

See #161, #88.