Hi, these are the default styles for hero subtitles:

.light-content .hero-subtitle {
    color: #fff;
    opacity: 0.5;
}

.hero-subtitle {
    font-size: 16px;
    color: #000;
    width: 100%;
    z-index: 10;
    margin-left: 3px;
    line-height: 30px;
    -webkit-transition: color 0.2s ease-in-out 0s !important;
    transition: color 0.2s ease-in-out 0s !important;
}

after you customize them just add the code to  theme options -> styling options -> custom CSS