Overriding the encapsulated CSS of external component

You can use the special css /deep/ instruction. See the documentation

So, if you have

app
  sub-component
    target-component
      <div class="target-class">...</div>

You can put in your apps css (or less):

/deep/ .target-class {
  width: 20px;
  background: #ff0000;
}

Obviously, you can put this css fragment in sub-component as well.