When using the Leatherbound theme, highlighting a course topic will change the colour of any basic text (i.e. hyperlinks are unaffected) from black on white to white on white. The topic title, summary and labels are therefore unreadable in a highlighted topic.

If the course is in weekly format, the section name (whether they be the dates or a custom section name) IS displayed correctly in a highlighted topic.

I would guess this is happening because when the course is in topics format, the topic number needs to change to white to contrast against the orange bars either side of a highlighted topic.