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    TL;DR: appearance: none;.
Checkboxes are hard to style. But when you're making a website, they may look ugly.
As you can see here, this bland checkbox does not fit into my clearly great website(/s). But really, it does not fit in at all.
The first step toward styling it how we want it is to give it an appearance of none, and a width and height that are what you want.
input[type="checkbox"] {
    appearance: none;
    width: 15px;
    height: 15px;
    background-color: grey;
}
Now we can do whatever we want to it. You're also able to add a :checked sudo to change certain elements(like the background-color) depending on if the checkbox is checked or not.
input[type="checkbox"] {
    appearance: none;
    width: 15px;
    height: 15px;
    background: #000;
    border: 1px #fff solid;
    border-radius: 2px;
}
input[type="checkbox"]:checked {
    background: #ce5aff;
}

 
             
            