How to edit WELCOME text in AIR Theme

It’s possible. There is no setting, but you can add some CSS in your theme or another new theme component:
For example:

.search-banner .custom-search-banner-wrap {
  h1 {
     /* CSS for the title */
     font-size: var(--font-up-6);
     color: aliceblue;  
  }

  p {
     /* CSS for the description */
  }
}