Hello,
Thanks for your Patience!
We have just checked this out and the Preloader appears to be displaying on Dark Background as expected. Please check the attached screenshot. If you are still facing an issue with this, any chances you can provide us with a Screenshot of the same so that we can look into this more closely. Thanks for your Patience.
Also, you can control the Preloader Colors with the following CSS:
[ch_pre type="css"].dark .css3-spinner {
background-color: #333;
}[/ch_pre]
Meanwhile, do let us know if we can help you with anything else or if you find any further issues with Canvas.