The nord theme is very nice but there is an issue with own posts which have a white background and white text. See screenshot
i see the same.
i changed the original nord.css and placed it in the external-themes folder and it picked it up! so that i could select 'nord2' as style from the dropdown box.
part changed
.mark, mark { padding: 0.2em; background-color: #474d58; }
nord2.css download link from my test site