I think that two years ago all text in the Preview was in white when using dark mode. According to xiota/geanylua-scripts#5 (comment), since last year some text is white and some text is black. The black text is hard to read.

I replaced in fountain.css this code:

/* Slug */
Dialog {
	break-inside: avoid;

by this code:

/* Slug */
Dialog {
	color: white;
	break-inside: avoid;

Now all the text in the Preview is in white.

What about the idea to add the line color: white; in fountain.css as default?
An alternative would be maybe color: yellow;, to give users a help where to change the color of dialogs.

I would also replace this code:

html {
	font-size: 8pt;
}

by this code:

html {
	font-size: 8pt;
	color: white;
}

It would be easier for users who have problems with CSS to know where to change the color for the rest of the text.