function changeStyle(){
 var paragraphe = document.getElementById('monParagraphe');
 paragraphe.style.backgroundColor="#000";
}
 
function load(){
 var buttonElt = document.getElementsByTagName('button');
 buttonElt.addEventListener('click', changeStyle)
}

<p id=’monParagraphe’>Lorem ipsum dolor sit amet, consectetur adipisicing
 elit, sed do eiusmod
 tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
 quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
 consequat.
</p>
<button onclick=changeStyle()>Changer le style</button>
 

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