CODE EXAMPLE FOR JAVASCRIPT

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')