Possible to change the patchwork background?

yes.
where you want to change background ?

in your style.css on line:1151 below style is coded.

.style-babycakes #masthead {
    background-image: url("images/patchwork3.jpg");
    border-bottom: 10px solid #774F47;
}

here you can change background-image property with your own.
You can change image as you required.

Are you want to set another image or want to set background color???

You can replace your existing code

.style-babycakes #masthead {
    background-image: url("images/patchwork3.jpg");
    border-bottom: 10px solid #774F47;
}

with

.style-babycakes #masthead {
    background-image: url("URL_TO_YOUR_NEW_BACKGROUND_IMAGE");
    border-bottom: 10px solid #774F47;
}

or if you want to use a solid color as background, you can replace your existing code with

.style-babycakes #masthead {
    background-image: none;
    background-color: HEX_COLOR_CODE;
    border-bottom: 10px solid #774F47;
}

first paste image in your
“wp-content/themes/patchwork/images/” directory.

than set

background-image: url(“images/here-your-image-name”);

image name in this property.
Make sure that your image name have no space…

hey save your image in ‘wp-content> theme >patchwork> images folder ‘.