How can I add {% if %} for a field name? What would be the correct way to write an {% if %}  "field_name" "is", in Drupal 8 Twig files?
I would like to add a CSS style specifically to one field and I believe this will achieve that.

Here is my current code within Twig file:

{%
  set classes = [
    'field',
    'field--name-' ~ field_name|clean_class,
    'field--type-' ~ field_type|clean_class,
    'field--label-' ~ label_display,
  ]
%}
{%
  set title_classes = [
    'field__label',
    label_display == 'visually_hidden' ? 'visually-hidden',
  ]
%}


{% if label_hidden %}


  {% if multiple %}
    &lt;div{{ attributes.addClass(classes, 'field__items') }}&gt;
      {% for item in items %}
        &lt;div{{ item.attributes.addClass('field__item') }}&gt;{{ item.content }}&lt;/div&gt;
      {% endfor %}
    &lt;/div&gt;
  {% else %}
    {% for item in items %}
      &lt;div{{ attributes.addClass(classes, 'field__item') }}&gt;{{ item.content }}&lt;/div&gt;
    {% endfor %}
  {% endif %}


{% else %}

  &lt;div{{ attributes.addClass(classes) }}&gt;
    &lt;div{{ title_attributes.addClass(title_classes) }}&gt;{{ label }}&lt;/div&gt;
    {% if multiple %}
      &lt;div class='field__items'&gt;
    {% endif %}
    {% for item in items %}

      &lt;div{{ item.attributes.addClass('field__item') }}&gt;{{ item.content }}&lt;/div&gt;
    {% endfor %}

    {% if multiple %}
      &lt;/div&gt;
    {% endif %}
  &lt;/div&gt;


{% endif %}
You copied field.html.twig to a new file, field--field-additional-text.html.twig. After clearing the cache, Drupal will only use this new file for that field only. Now you do not need logic in either template.

So you now can make changes, for example:

{% for item in items %}
  &lt;div class="additionalbreaker"&gt;{{ item.content }}&lt;/div&gt;
{% endfor %}


^ This means, for every item in this field (single or multiple values), print each value wrapped in this div, with this class. However, within a field template, you cannot output different fields as one, the sole purpose of a field template is to render the current field.

Read up on theming Drupal fields for more information.