The defaults follow classic browser colors.

KeyDefaultPurpose
text_color#000000Body text
link_color#0000eeUnvisited links
link_visited#551a8bVisited links

Override them in config.toml.

[extra]
text_color = "#000000"
link_color = "#cc0000"
link_visited = "#880000"

Links stay underlined by default. That is intentional: readability comes first.