Avatar images overlapping sticky header menu

The avatar images are overlapping my sticky header menu as I scroll through the comments area (see example link).

Separately, despite changing the ‘submit’ button text colour (to white), it isn’t reflecting on the front end. I’m able to change the background colour of the button, so I’m confused as to why this doesn’t work. Any help would be much appreciated!

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