How to change background color for whole template?

Hello,

I don’t know much php code and styling but I do things correct by finding answer on google.

I have a website which have white background I want to change it to light green.

In admin > editor, I can only see style.css. No default or any other .css file.

in this “style.css” file its blank so please tell me anyone how can I change background color of my website

Thanks

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