New send button icon and small mixin UI tweaks

- New send button icon

- remove double border between mixins

- match background color of collapsible headers (mixin__name) with
headers in Chrome inspector

- match disclosure triangle color with Chrome inspector and add some
white space

![old_new](https://f.cloud.github.com/assets/3692/1102528/db968044-182b-11e3-8efd-44b72b8795c9.png)