Declare it with two dash --. Usually define in :root, and it kebab-case
Use it with the function -var()
:root {
--my-color: red;
}
div {
color: var(--my-color);
}
The var() function can take a second argument, that become the
fallback value in case the variable failed.
Can be nested with another var().
div {
color: var(--my-color, blue);
background-color: var(--my-color, var(--other-color, red));
}
The scope of a variable include the selector where it is declared, and its descendants.
Setting them in :root make them accessible in the whole document.