SES-2092 Removed default PNG images, and replaced with nearly identical CSS specifications.
SES-2092 Tweaked the gradient to be closer to PNG gradient.
SES-2092 These changes rely on IE 10+, so keeping stylesheets supporting IE 5.5-7 is silly.
SES-2092 Added pre-IE10 filter property to get something close to the desired gradient on those IE versions.
SES-2092 Restored hr_menu PNG, since it actually contributed to the look.
Merged in dvisser/sesame/SES-2092-replace-png-with-css (pull request #304)

SES-2092 Replaced background PNG files with CSS color and gradient properties