Ok I have fixed the problem for myself.
Black image background problem comes from browser engine itself. Chromium-based browsers have same problem.
The disappointing part is that ViewEnhance extension on the MX extension page is out of date (last updated on 2018); however, the author has recently updated version on GitHub (Link). Install the latest version for the fix. Author mentions "Install from Maxthon Extension Center, or newer versions from the archive, because as it seems Maxthon doesn't really review extension updates anymore". It is quite pathetic. Addons/Extensions are quite important for browser and its users. Not only Maxthon has failed to attract more authors and add new extensions but also failed to appreciate existing authors.
I hope all this changes with MX6.