Caption Color on Header Image

Please remove your coming soon plugin so we can see the actual site.

ONe option — leave it white but use a text shadow:

.frontpage-banner .caption h2 a {
    text-shadow: 1px 1px #000;
}
.frontpage-banner .caption .read-more {
    text-shadow: 1px 1px #000;
}

or go for just black

.frontpage-banner .caption h2 a {
    color: #000;
}
.frontpage-banner .caption .read-more {
    color: #000;
}

To add CSS: use the “Additional CSS” option in the customizer. https://codex.ww.wp.xz.cn/CSS#Custom_CSS_in_WordPress

Learn to use the Chrome Developer Tools to help you see and test changes to your CSS.