create "neutral" submit button style

Right now green buttons generally mean "submit/yes/add/continue" and red buttons generally mean "cancel/no/delete." This usage seems reasonable at this point. We do have places though where there is a "neutral" action, like "Invert Selection" sitting next to the "Add" button. It would be better to have a grey or blue button style for these kind of things, rather than two greens setting side by side.