Thanks for the feedback!

The image menu on the left is static, and is hard-coded into the template. It consists of one image for each main section page, and the hover effect is done in the css file using opacity. The images are in the media folder, so if I have to add another main section, all I have to do is upload the new image to the media folder, then use the template editor to add it to the template, rather than having to uninstall/reinstall the whole theme.
I had hoped to find a way to do it dynamically, but wasn't able to.
Doc - I've made the container background color white now. Could you please check it again and see if that is better?