[a11y] Force body background-color to white

For accessibility reasons, some users change the default background color and foreground color in their pref's browser.
The a11y rule says that we have to always set a background color if we set a foreground color (and vice versa). In Uno's case, we change the black font color with dark gray, so we need to overwrite, or force to be more accurate, the white body background so that the contrast remains fine.