The body is not filling the entire screen, how can I solve?

Question

Hey, guys. All right?

I’m doing an exercise with form and background body was made with linear-gradient, but somehow it seems that the body is "stuck" in the form container. If I change the height of the container it increases or decreases the limits of the body. All the Divs are closed correctly and the code is very simple.

body {
    font-family: 'Trebuchet MS', Helvetica, sans-serif;
    background: linear-gradient(to bottom, #8C11E1, #EA8195) no-repeat;
}

#container {
    width: 400px;
    height: 300px;
    background-color: rgba(255, 255, 255, 0.3);
    border-radius: 10px;
    margin: 0px auto;
    margin-top: 120px;
    text-align: center;
}

This exercise is being done with simple CSS, so I wonder if there’s anything connected to screen responsiveness and everything, only this has never happened before, so I don’t know how to fix it. You can help me? :/