If you want to change Carousel header text style
You can follow these.
#1. Carousel header text color
You can access Site Styles > Colors > Find this option
You can also use this code to Custom CSS to change its color.
span.summary-header-text { color: #f1f !important; }
#2. Carousel header text style (font, size..)
You can use this to Custom CSS box
span.summary-header-text { color: #f1f !important; font-size: 30px; font-family: monospace; letter-spacing: 3px; line-height: 30px; }
Result