CrouchingBruin on "Need help changing post title text for mobile visitors (CSS)"

The selector you want to use is .article .post-title, but you also need to enclose it in a media query. Media queries are what makes themes responsive. You set CSS rules depending upon the width of the screen. For example, these media queries changes the font size to 20px when the viewport is 768px wide (about a tablet width) and 14px when the width of the view port is 480px or narrower:

/* Tablet: 768px */
@media only screen and (max-width: 768px) {
   .article .post-title {
      font-size: 20px;
   }
}
/* Phone: 480px */
@media only screen and (max-width: 480px) {
   .article .post-title {
      font-size: 14px;
   }
}

I picked those width values because your theme currently has media queries at those points.