I'm talking out of ignorance here but are you absolutely sure the ActiveX problem is related to the use of a fallback for the background
property with a custom property value? Are you using any add-ons for IE? If you are, have you tried disabling them?
Some add-ons depend on ActiveX and they can cause some issues when rendering a site if they're not working properly.
It strikes me as odd that a CSS fallback property could lead to this case, since most layout engines just ignore properties with invalid values, i.e.:
.class {
color: #000;
color: invalidValue;
}
Since invalidValue
isn't a valid color value for the color
property, it is a common practice for layout engines to ignore this property (and its value) and just use the previous declaration of that property (if there isn't a previous declaration, they fallback to the initial/inherited value). With this being said, if IE would have some sort of problem with your --main-bg-color
custom property it should use brown
as the fallback, as it was initially intended.