Green Stimulus theme intro text size

I am very new to WordPress. I’m using the Green Stimulus theme. I’ve been able to get it setup for the most part, but I can’t find a way to change the font size of the intro text (which is directly below the slider). I’ve researched specific code and followed the theme’s instructions to alter the custom css file. No changes made in that file work; even tried !important as another site recommended. I refresh my page and nothing happens. The intro text is way too large and looks bad. I’ve wasted hours trying to figure out what else to do!!