When using a custom variable in CSS, the value is the value that is used when the given variable is invalid. code example

Example: css variables

/* "css variables" */
/* not for ie but good for Edge and other browser :D */

/*
 By declaring a custom property on the :root pseudo-class
 and using it where needed throughout the document
*/
:root {
  --primary-bg: #8a2be2;
  --btn-font-size: 18px;
  --btn-padding: 10px 15px;
}

.btn-primary {
  background-color: var(--primary);
  font-size: var(--btn-font-size);
  padding: var(--btn-padding);
  color: #E2E2E2;
}