Checkbox problem in post editor

When inside the post editor, we cannot see the ticks inside the category checkboxes. They are visible but incredibly feint. This seems to be the case on all browsers except for IE

It has only started recently, so I presume it is as a result of a recent plugin update

Checkboxes outside of the post editor (e.g in settings) are fine .. and you can even see them fine when you “quick edit” a post. It is only inside the usual full page post editor.

The page I need help with: [log in to see the link]