After some googling, I found out how to use the custom CSS editor in Wordpress to make my Jetpack-theme Dark Mode enabled.
@media (prefers-color-scheme: dark) {
/* Overrides for Dark Mode. */
html, body {
color: white;
background: black;
}
.site-title {
font-family: ‘Playball’, cursive;
font-size: 28px;
font-style: italic;
color: #FFF;
text-shadow: 0 0 4px #FFF;
}
.site-description{
font-size: 15px;
font-style: italic;
color: #FFF;
text-shadow: 0 0 4px #FFF;
}
}