You're using strange syntax.

var changeTextForBundle = document.querySelector('.picky-bundler-container .picky-custom-html h2');
changeTextForBundle.style.backgroundColor='#363636';

should work better ;)

I'm not certain if you have a problem with the CSS selector in querySelector, but you may want to use querySelectorAll to select all element matching the selectors.

What doesn't look right is trying to set the background-color in your JavaScript code. Change the background-color setting in JavaScript with backgroundColor:

var changeTextForBundle = document.querySelector('.picky-bundler-container .picky-custom-html h2');      
changeTextForBundle.style.backgroundColor = '#363636';

If you would like to use the querySelectorAll option, the following code would perform the same task for all elements matching your selector string:

var changeTextForBundle = document.querySelectorAll(
  ".picky-bundler-container .picky-custom-html h2"
);
changeTextForBundle.forEach(function (element) {
  element.style.backgroundColor = "#363636";
});

Runnable example

var changeTextForBundle = document.querySelectorAll(
  ".picky-bundler-container .picky-custom-html h2"
);
changeTextForBundle.forEach(function(element) {
  element.style.backgroundColor = "#363636";
});

<div class="picky-bundler-container picky-widget">
  <div class="picky-custom-html">
    <h2 style="width:100%; display:flex; justify-content:center; align-items:center;">Frequently Bought Together</h2>
  </div>
</div>