Regarding solo/mute buttons, there must be found some compromise between style and usability. For good usability, these buttons should have some bigger colored area, not just outlines. If you put the "S" inside as transparent/black, the button fits a bit better. I have tried some(all?) combinations with emphasised cornes, but did not like any of those. Notice also that this button of mine is 2px narower than the outline of the mute button above.