line number is black for ANSI theme

If I understand it correctly, there’s only one style for ANSI output format (so --style=<theme> is ignored). This is fine, but the default theme sets the line number as black. This works well in terminals with light-color background, but in dark background the line numbers would be invisible. Shall we change it to use default color for line numbers? This way the light-terminal users won’t notice any difference, while dark-terminal users will have usable line numbers. The “default text color” respects user preference (generally light color on dark background, or dark color on light background).