I'm looking for the same kind of solution, but that code does not work for me at all. I am using the Miller template, the site is www.johnlynchspeaks.com. I tried the above code and it doesn't do anything. The code I've been using is as follows but it has some issues:
<script>
window.onload = function()
{document.getElementsByClassName('Header-branding-logo')[0].src = 'https://static.wixstatic.com/media/2279fb_db1043088564425bb18696d17a327498~mv2.png/v1/fill/w_343,h_134/JLS-logo-light.png';
};
</script>
The problem I have is that this code doesn't "trigger" for some reason unless you refresh the page—so the light logo shows up on the homepage when you first visit the site, but on the other pages where the dark logo is supposed to show up, it doesn't unless you refresh the page. And the same thing happens if you first visit the site from a subpage—the correct dark logo shows up, but then if you navigate to the home page, the dark one shows there as well until you refresh the page.
Anyone have a fix?