CSS default border color

Let we have the following html markup:

<div id="parent" class="parent">
    <div id="child" class="child">
    </div>
</div>

and corresponding css styles:

.parent{
    border-style: solid;
    border-color: green;
    border-bottom: solid 10px;
    background:grey;
    width: 300px;
    height: 300px;
    padding: 10px;
}
.child{
    border: 20px solid;
    background: aqua;
    height: 50px;
    margin: 10px;
}

.parent {
  border-style: solid;
  border-color: green;
  border-bottom: solid 10px;
  background: grey;
  width: 300px;
  height: 300px;
  padding: 10px;
}
.child {
  border: 20px solid;
  background: aqua;
  height: 50px;
  margin: 10px;
}

<div id="parent" class="parent">
  <div id="child" class="child">
  </div>
</div>

We can see that child's border color is black, but i dont define this color explicitly.

How I can change this default color to green?