I've just added sample CSS, to show how it works, for your particular case you need to figure it out by yourself, you can use browsers inspector to find element to apply CSS for.

Please give us a hint where to change react default background color. ty

I thought that you have some custom CSS styles, but it looks like it's bug in Decorous template:
https://github.com/unaio/una/issues/2481

Generally to change this background NO changes in react are needed it's on the site, you can use inspector in browser's developer tools to check the element with this background and change it with custom CSS, it should be pretty easy if you have basic knowledge if HTML and CSS.