Just like in print, don’t use the darkest black available in your body text (#000). Instead, go for something softer, like #444. You can use a slightly darker color for titles like (#222) for example. Since these values are considered a de facto standard in practice, they're not necessarily ordained by law.
$body-copy-color: #444 !default;
$heading-color: #222 !default;
body {
color: $body-copy-color;
}
h1,
h2,
h3,
h4,
h5,
h6 {
color: $heading-color;
}