Using :Focus to Style Outer Div

Using :focus to style outer div?

While this can't be achieved with CSS/HTML alone, it can be achieved with JavaScript (without need of a library):

var textareas = document.getElementsByTagName('textarea');

for (i=0;i<textareas.length;i++){
    // you can omit the 'if' if you want to style the parent node regardless of its
    // element type
    if (textareas[i].parentNode.tagName.toString().toLowerCase() == 'div') {
        textareas[i].onfocus = function(){
            this.parentNode.style.borderStyle = 'solid';
        }
        textareas[i].onblur = function(){
            this.parentNode.style.borderStyle = 'dashed';
        }
    }
}

JS Fiddle demo.

Incidentally, with a library, such as jQuery, the above could be condensed down to:

$('textarea').focus(
    function(){
        $(this).parent('div').css('border-style','solid');
    }).blur(
    function(){
        $(this).parent('div').css('border-style','dashed');
    });

JS Fiddle demo.

References:

• getElementsByTagName().
• onfocus.
• onblur.
• parentNode.
• tagName.
• toString().
• toLowerCase().
• style.
• focus().
• blur().
• parent().
• css().

How to add outer colour around this div when it has focus?

As Paulie_D advised, outline property is what you're looking for when the element is focused. If you need to know what options are available for the outline property, W3 Schools has great visuals that you can review

Make :focus change css of another class

Using pseudo-classes (such as :hover or :focus) to modify other elements can only be done if the other elements are siblings or children of the element which has the pseudo-class. That's because CSS child/sibling selectors are fairly restrictive.

You can use the > selector to select a direct child, and the + selector to select a direct sibling. For example, if you have the following HTML:

<form>
    <input type="text" />
    <input type="submit" />
</form>
<p class="arbitrary">
    This is an arbitrary element. It is neither a child nor sibling of 
    the text field. It cannot be selected as a result of a pseudo-class 
    action on the textfield using CSS, but can be selected using 
    client-side scripting such as JavaScript.
</p>

You could style the button when the text field has focus (because it is a direct sibling of the text field), but there is no possible way to style the arbitrary paragraph as a result of the text field receiving focus (because it is neither a child nor sibling, it is the sibling of a parent) without using client-side scripting (JavaScript, jQuery, etc.).

This CSS would style the submit button, and can be altered to select any direct or indirect child or sibling:

input[type="text"]:focus + input[type="submit"] {
    /* some sweet CSS */
    background-color:green;
}

Using Javascript, of course, you have much greater flexibility. The focusin and focusout events can be used to toggle CSS classes. Here's an example that demonstrates both the CSS and JavaScript techniques of achieving this.

function setFocused() {
  document.querySelectorAll('.arbitrary').forEach((result) => {
    result.classList.add('focused');
  });
}

function unsetFocused() {
  document.querySelectorAll('.arbitrary').forEach((result) => {
    result.classList.remove('focused');
  });
}

document.querySelectorAll('input[type="text"]').forEach((result) => {
  result.addEventListener("focusin", setFocused);
  result.addEventListener("focusout", unsetFocused);
});

input[type="text"]:focus + input[type="submit"] {
  /* some sweet CSS */
  background-color: green;
}

.arbitrary.focused {
  /* even more sweet CSS */
  color: red;
}

<form>
  <input type="text" />
  <input type="submit" />
</form>

<p class="arbitrary">
  This is an arbitrary element. It is neither a child nor sibling of
  the text field. It cannot be selected as a result of a pseudo-class
  action on the textfield using CSS, but can be selected using
  client-side scripting such as JavaScript.
</p>

.explore-card-hidden-info {
    display: none;
}

.explore-card-light:focus + .explore-card-hidden-info {
    display: block;
}

<div>
  <div class="explore-card-light web-box-hor" style={props.style} tabindex="1">
    <div>
      <img src={props.src} alt="imgex" style={{width: "100%", margin: 'auto', height: 'auto'}}></img>
    </div>
    <div style={{marginLeft: '20px'}}>
      <p class="card-title" style={{textAlign: 'left'}}>{props.title}</p>
      <p class="card-main-text">{props.main}</p>
      <p class="card-footer-text">{props.ex}</p>
    </div>
    <div style={{marginLeft: '20px'}}>
      <p class="explore-card-footer-text" style={{margin: 'none'}}>rates: {props.rate}</p>
      <p class="explore-card-footer-text" style={{margin: 'none'}}>views: {props.views}</p>
      <p class="explore-card-footer-text" style={{margin: 'none'}}>purchases: {props.buys}</p>
    </div>
    <div style={{ padding: '30px 150px' }}>
      <p class="card-title">{props.money}</p>
      <OpBtnOutlineRound text="Buy"></OpBtnOutlineRound>
    </div>
  </div>
  <div class="explore-card-hidden-info">
    <h2>
      I adm hidden!
    </h2>
  </div>
</div>

.cool {  font-size: 30px;  margin: auto;  outline: none;  padding-bottom: 0.2em;  border-width: 0px 0px 2px 0px;  border-bottom-color: black;}
div.bar {  width: 365.3px;  height: 2px;  background-color: #42c0fb;  position: relative;  top: -2px;  display: none;}
input:focus + .bar {  display: block}

<div style="margin: auto;">  <input class="cool" type="text" />  <div class="bar"> Hello</div></div>