If the 'default' style isn't set to a named style, then probably the
filetype styles have been overridden with manual colors and may not
work properly with color schemes.

Note: HTML-based filetypes won't show a warning because they don't
have a 'default' style (they use 'html_default' instead).

This is a simple heuristic, but is worth having to avoid some
spurious bug reports when using the color schemes dialog.