useCssVar 
Manipulate CSS variables
Demo 
Usage 
ts
import { useCssVar } from '@vueuse/core'
import { useTemplateRef } from 'vue'
const el = useTemplateRef('el')
const color1 = useCssVar('--color', el)
const elv = useTemplateRef('elv')
const key = ref('--color')
const colorVal = useCssVar(key, elv)
const someEl = useTemplateRef('someEl')
const color2 = useCssVar('--color', someEl, { initialValue: '#eee' })Type Declarations 
ts
export interface UseCssVarOptions extends ConfigurableWindow {
  initialValue?: string
  /**
   * Use MutationObserver to monitor variable changes
   * @default false
   */
  observe?: boolean
}
/**
 * Manipulate CSS variables.
 *
 * @see https://vueuse.org/useCssVar
 * @param prop
 * @param target
 * @param options
 */
export declare function useCssVar(
  prop: MaybeRefOrGetter<string | null | undefined>,
  target?: MaybeElementRef,
  options?: UseCssVarOptions,
): ShallowRef<string | undefined, string | undefined>