title: Disable submit button and change CSS of button if form not filled
body: <p>I have one input box number and button.</p>
  
  <p>On page load the button is greyed out. CSS applied is "<code>button button--primary button--primary--dark button--full-width button--disabled</code>"</p>
  
  <p>When I enter something in input box the CSS for button should change to "<code>button button--primary button--primary--dark button--full-width</code>"</p>
  
  <pre><code>import React, { Component } from 'react'
  
  class NumberCheck extends Component {
  
      render() {
  
          return (
              &lt;form className="spring spring--sm"&gt;
                  {/* &lt;div className="errorMsg"&gt;{this.state.errors.mobileno}&lt;/div&gt; */}
                  &lt;div className="alert alert--light alert--warning validation__warning"&gt;
                      &lt;div className="caption"&gt;
                          &lt;div className="caption__media caption__media--top alert__media"&gt;
                              &lt;svg className="icon icon--extra-small icon--inherited" data-vft="icon-modifiers"&gt;
                                  &lt;use xmlnsXlink="http://www.w3.org/1999/xlink" xlinkHref="#icon-block" /&gt;
                              &lt;/svg&gt;
                          &lt;/div&gt;
                          &lt;div className="caption__text caption__text--top alert__text"&gt;
                              Please enter valid mobileno
                          &lt;/div&gt;
                      &lt;/div&gt;
                  &lt;/div&gt;
                  &lt;input type="number" className="form__input form__input--dark" name="mobileno" min={8} max={13} /&gt;
                  &lt;div id="submit" className="form__row form__row--medium gutter--bottom"&gt;
                      &lt;input type="submit" className="button button--primary button--primary--grey button--full-width button--disabled" value="Continue" /&gt;
                  &lt;/div&gt;
              &lt;/form &gt;
          );
      }
  }
  
  export default NumberCheck;
  </code></pre>