Hello Mat,

The fullscreen slider page does not hold any content it displays only the portfolio items you have defined in admin.

To change the styles of the titles in the slider use

.slide-title {
    font-size: 80px;
    line-height: 90px;
    color: #000;
    font-weight: 600;
}

in customizer -> additional CSS, change the values for font size and font weight, line height