Using Style Properties > Buttons > Button focus/hover/active I can make changes to the Upload and More buttons without any issue, however the "normal" light blue color of the Post Reply button doesn't seem to exist in style properties.
I've tried background-color changes on .submitUnit .button.primary and .button.primary and can change the color (using EXTRA). The problem is that this blocks the focus/hover/active functions for the Post Reply button. I think it needs something like .button.primary .inactive ???