Custom background for Chrome new tab replacement extension - css

Question

I'm developing a new tab replacement extension for Google Chrome and I'd like to allow the user to customize the background, to do so I'm using the storage.sync API as suggested by this page.
The problem is that the style changes are applied asynchronously, so the default background (white) is briefly used during the page load resulting in unpleasing flashes.
Possible (unsatisfying) solutions are:
do not allow to change the background;
hard code a black background in the CSS (and move the problem to custom light backgrounds);
use a CSS transition (still super-ugly).
What could be an alternative approach?
Follows a minimal example.
manifest.json
{
"manifest_version": 2,
"name": "Dummy",
"version": "0.1.0",
"chrome_url_overrides": {
"newtab": "newtab.html"
},
"permissions": [
"storage"
]
}
newtab.html
<script src="/newtab.js"></script>
newtab.js
chrome.storage.sync.get({background: 'black'}, ({background}) => {
document.body.style.background = background;
});

Answer 1

I come up with a reasonable solution. Basically since the localStorage API is synchronous we can use it as a cache for storage.sync.
Something like this:
newtab.js
// use the value from cache
document.body.style.background = localStorage.getItem('background') || 'black';
// update the cache if the value changes from the outside (will be used the next time)
chrome.storage.sync.get({background: 'black'}, ({background}) => {
localStorage.setItem('background', background);
});
// this represents the user changing the option
function setBackground(background) {
// save to storage.sync
chrome.storage.sync.set({background}, () => {
// TODO handle error
// update the cache
localStorage.setItem('background', background);
});
}
This doesn't work 100% of the times but neither do the simple:
document.body.style.background = 'black';
So it's good enough.¹
¹ In the real extension I change the CSS variables directly and I obtain much better results than setting the element style.