get css custom property javascript
/* This will return the value of the --color custom property
from the root element */
getComputedStyle(document.documentElement).getPropertyValue('--color')
const customButton = document.querySelector('.custom-btn')
/* This will return the value of the --color custom property
from the root element since it is inherited down to the
customButton element. */
getComputedStyle(customButton).getPropertyValue('--color')
/* This will return the value of the --background-color
custom property from this customButton element */
getComputedStyle(customButton).getPropertyValue('--background-color')