If you want to apply on all buttons and not only sidebar widget buttons then instead of above CSS code please use following custom CSS code.

#page-container button,
#page-container input[type=button],
#page-container input[type=reset],
#page-container input[type=submit] {
    border-radius: 17px;
    padding: 1px 10px;
    background: #e9cbea;
    color: #000;
    border-color: #aeb0d4;
}

You can change the color value in the above code to whatever you want to use by referring the following pages.

http://www.w3schools.com/html/html_colors.asp
http://www.w3schools.com/html/html_colorvalues.asp
http://www.w3schools.com/tags/ref_colorpicker.asp