Feature/highlight feedback messages

I often feel like students are oblivious to the feedback messages from incorrect answers. This started out as a way to make those message pop out more by giving them some color. While I was in there I changed some other little things about the css in the math4 theme.

Try Submitting correct answers, incorrect answers, answers that generate a feedback message, and then also click the Preview and Check buttons. Aside from these things, td and th elements in the results table are now vertically aligned middle, and the red that is used with white background in ResultsWithError messages is now a darker shade.