Please don't use Chrome to workaround this.
It's just that
html.composing body.page-topic.page-topic-13568.page-topic-dark-theme.page-topic-category-10.page-topic-category-nodebb-themes.page-status-200.skin-paper div.composer.resizable.reply div.composer-container div.row.write-preview-container div.write-container textarea.write doesn't have
background: white, as you can see in the excellent Firefox DOM inspector. In the NodeBB stylesheet,
color: inherit, but no
background: inherit. So the browser stylesheet for
textarea's 'background' is used, which is the theme background by default, which is dark in this case.
Some digging shows that these values come from normalize.css 3.0.2. Seems like this is the same problem as this one: https://github.com/necolas/normalize.css/issues/469
This has been fixed in normalize.css in Jan 2016 by removing
color: inherit. In the case of a dark background (coming from the Gtk theme), the text should now be white (or whatever the Gtk theme says) instead of dark, which makes the text readable.
Suggested fix: use a version of normalize.css later than Jan 2016
PS: Maybe Chrome is just not able to show the dark Gtk theme? Could be an enhancement request