Ah, I think I might understand what you’re trying to do. Could you try this CSS:
body.custom-background #page {
background-image: none;
background-color: #ffffff;
}
That should let you upload your own custom background image but then reset the background colour of the main column to white – just change the colour to anything you like.
Don’t edit the theme files directly, otherwise your changes will be overwritten whenever the theme is updated.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.
Let me know how it goes!