It's getting overridden by the Day.css stylesheet. But you should be aware that all of your changes are going to be erased when you update the theme - -that's why I suggested a custom CSS plug-in or using a child theme.
Then change the above to this:
#content img {
background: none !important;
border: none !important;
border-radius: none !important;
padding: 0 !important;
}