Getting Multiple items from Chrome storage?

As per the official documentation it will be as below.

chrome.storage.sync.get([
 'customImage',
 'customColor',
 'customRandColor',
 'customRandImage'
], function(backgroundCheckedOptions) {
 document.getElementById('optionsCustomImage').checked = backgroundCheckedOptions.customImage;
 document.getElementById('optionsBackgroundColor').checked = backgroundCheckedOptions.customColor;
 document.getElementById('optionsRandomColor').checked = backgroundCheckedOptions.customRandColor;
 document.getElementById('optionsRandomImage').checked = backgroundCheckedOptions.customRandImage;
});

From the Chrome Storage documentation, it says:

StorageArea.get(string or array of string or object keys, function callback)

Easiest would be to pass an array by replacing your {} with []