What’s shown in the video is an animated transition from a larger to a smaller font size. It’s caused by the following CSS in the Marfa theme:
nav.main-nav a, #footer a, #post-nav a, p a {
/* … */
transition: all .35s;
transition-timing-function: cubic-bezier(.7, 0, .3, 1);
}
The dark mode version of Marfa explicitly disables animation with transition: none !important; and that’s why the flickering goes away when you activate dark mode.
As to why this is only happening in Safari, it might actually be an implantation bug in that browser. But to know for sure, one would have to analyze the CSS in more detail.