No way to set a theme color to the default foreground color

[Nemo157]

crossterm::style::Color only supports the 8-bit ansi color codes, there is no way to make it emit code 39 which uses the default foreground color (which can be distinct from the normal and bright variants of both white and black).

As a workaround, adding support for parsing a string like reset to Color::Reset should give a similar result.