A customer liaison sent a question to the shell team.
Subject: Assistance [REG:628318530717959]
Our customer has created a custom theme by editing one of the standard High Contrast themes, but it doesn't seem to be working quite right. For example, Internet Explorer doesn't show Web page background images. How can we fix this? Details from the customer are attached.
This sounds like a customer is tinkering around doing some personalization, and now they're using their "unlimited support" contract to get some help to get their super-l33t theme to look even more awesome. In the attachment, the customer explained that the default is so plain, so they took the High Contrast theme and made it more colorful. Personally, I think their new theme looks hideous, with garish colors everywhere, but hey, Hot Dog Stand is a thing, so who am I to say.
Anyway, the reason that Internet Explorer doesn't show Web page background
images is that you are using a high contrast theme,
that maybe doesn't have as much contrast as
And since it is a high contrast theme,
applications will ask, "Is a high contrast theme active
and the operating system will say "Yup".
Or they'll ask
"Should I show overlapping content
and the operating system will say "Nope."
and applications will say,
I'll better remove background images,
textured backgrounds, watermarks, and any other content
that would interfere with the primary content.
I hope that helps."
In other words: The reason the backgrounds are gone is that you started with an accessibility theme, so you're getting all the accessibility side effects. If you want to create your Hot Dog Stand theme, and you don't want the accessibility baggage, then start with a theme that isn't an accessibility theme.
Bonus reading: Building a more accessible web platform.