How i can customize my checkbox with css and Bootstrp?

Im crating a form with many fields and a checkboxes, the problem is that im trying to customize the checkbox(unsuccessfully) according with the style of my form.

As you can see in the picture, the tick of the checkbox is black, and i need it green. How i can change the color? How i can change the size of checkbox?

This is my HTML:

        <div id="Cajamitad1">
            <form role="form">
                <div class="checkbox">
                    <label><input type="checkbox" value="" >
                    <a class="FontStyle">Vegano</a>
                    </label>
                </div>
        <       div class="checkbox">
                    <label><input type="checkbox" value="">
                    <a class="FontStyle">Diabetico</a>
                    </label>
                </div>
                <div class="checkbox">
                    <label><input type="checkbox" value="">
                    <a class="FontStyle">Celiaco</a>
                    </label>
                </div>
            </form>
        </div>

and this my CSS:

#ContForm{
    display: inline;
    box-sizing: border-box;
}
.checkbox{
    margin: 10px;
    padding: 10px;
}

.checkbox input[type="checkbox"]{
    margin-left: 5px;
}
#Cajamitad1{

    margin-top: 10px;
    width: 40%;
    float: left;
    padding-left: 200px;
}