CSS Variables https://developers.google.com/web/updates/2016/02/css-variables-why-should-you-care?hl=en
Differences between CSS variables and Preprocessor variables: https://philipwalton.com/articles/why-im-excited-about-native-css-variables/
root {
--main-color: #06c;
}
#foo h1 {
color: var(--main-color);
}