The original issue this bug is about was actually related to course completion and not activity completion, so I am adding that and reassigning. (You could have activities with automatic completion, but which are not required for course completion; e.g. a practice quiz, that type of thing.)
Regarding the issue with the icons, which IS activity completion - that's kind of a separate problem. The icons are bitmaps so there is no need for CSS. We actually have a new set of icons for the activity completion at the OU which are nicer (they are drawn by a graphic designer as opposed to... er... by me). The 'automatic completion, not complete' icon is now visible with a dotted line border. However, the change to these icons might confuse users as they are visually somewhat different, so it would certainly only be appropriate for a major Moodle version - 2.2 now.
If you would like the icons improved, could you please file a separate bug about that, and then maybe post about it on a suitable forum to see what people think? In case people hate the new icons I don't want to change it and annoy everyone... If you file that bug (in 'Activity completion' category) and remind me, I'll make a screenshot showing the new icons or something.