Here's how I would inline it, and I encourage people to do it this way. Intermediate variable is not necessary. Ternary is not necessary since false evaluates to null in jsx.

  render() {
   return (
     <div className='card'>
       {this.props.delta > 0 &&
         <strong className={this.props.delta > 0 ? 'text-success' : 'text-danger'}>
           {this.props.delta}
         </strong>
       }
       {this.props.title}
     </div>
   );
  }