in javascript you can make but please describe more

div.style.border="1px solid #000";

With pure html/js it would be look like that:

.showroom-card {
  width: 500px;
  height: 300px;
  background-color: yellow;
  border: 2px solid red;
}

.selected {
  border: 2px solid black;
}

  <div
    id="card1"
    class="showroom-card"
    onClick="(function(divId) {
      targetDiv = document.querySelector(`#${divId}`)
      targetDiv.classList.toggle('selected') 
    })('card1');return false;"
  ></div>

But in react you must use state of component to manipulate div's style. For example, you would use divToggled variable in state of your component to render border and manipulate its color. The handler function, named handleDivClick change state and component will be rerendered:

class YourComponent extends React.Component {
  ...
  handleDivClick = () => {
    this.setState(divToggled: !this.state.divToggled)
  }

  ...
  render() {
    return (
      <div
        onClick={this.handleDivClick}
        className={`showroom-card ${this.state.divToggled ? 'selected' : ''}`}
      />
    )
  }
}