unable to add a border to popup modal

I am using a Popup modal component from the reactjs-popup library.

export default () => (
  <Popup
    trigger={<button className="button"> Guide </button>}
    modal
    nested
  >
    {(close: any) => (
      <div className="modal">
        <button className="close" onClick={close}>
          &times;
        </button>
        <div className="header"> Guide </div>
        <div className="content">
          {' '}
      
          <img src={img} alt="charts" className="charts" />
          What is overfitting?
          <br />
          What is underfitting?
          This example demonstrates the problems of underfitting and overfitting and how we can use linear regression with polynomial features to approximate nonlinear functions.
        </div>
        <div className="actions">
        </div>
      </div>
    )}
  </Popup>
);

This is how it looks like:

I want to add a thick black border to the popup component. How can I do so? My current css doesnt seem to work.

.modal {
    font-size: 12px;
    border-color: black;
    border-radius: 100px;
  }

Answer

You need to set border-style: solid; the default is none. (maybe border-width too)