I want to send the value of Data- * to CSS

In a clear way:

if the value is Entered send it to css and if the value is not entered send the default value. so i tried that:

<whatsapp data-theme="black"></whatsapp>

let whatsapp = document.querySelectorAll("whatsapp")

if(whatsapp.dataset.theme == null ||whatsapp.dataset.theme == undefined ) 
{
  whatsapp.dataset.theme = "whatsapp-background.png"
  
} 

whatsapp.style.background = "url('"+whatsapp.dataset.theme+"')"
whatsapp.style.background = whatsapp.dataset.theme