With Firefox 64 and the Adwaita Dark GTK+ skin the term box editing fields are white text on white background (see screenshot).

This is caused by a background: none; rule that resets the textarea background color from black to white but is not followed by a "color: inherit;" rule to set the text color from white to black.
I believe the css should be:

background: none;
color: inherit;

The affected files seems to be:
view/resources/jquery/wikibase/themes/default/jquery.wikibase.sitelinklistview.css
view/resources/jquery/wikibase/themes/default/jquery.wikibase.entitytermsforlanguagelistview.css