I started studying CSS and I'm using an online IDE called replit to do some tests.
Today I've tried to create a simple rectangle shape and I realized that it couldn't reach the page borders, so I'm here to ask how to make this rectangle to reach it (https://2.thiagoalonso05.repl.co).
I created an empty div and put its class as greybar, after that, I went to CSS. Here's my code:
body {
    background-color: #181818;
}
.greybar {
    display: block;
    height: 48px;
    background-color: #202020;
}
Here's an image showing where I want the grey bar to reach (the red part):
