The move to less-plugin-autoprefix in 53bc220 exposed several linear-gradient() calls that were being formulated incorrectly, resulting in strangely rendered buttons in Firefox and IE.  The <side-or-corner> parameter should be expressed in "to <side-or-corner>" syntax rather than the old "<side>" syntax without the "to".
Changed all usages to "to" syntax.  For the vertical-three-colors case added an explicit "to bottom", even though this was already the implicit default.
Also fixed an unrelated typo in defs.css to change "rgpa" to "rgba".

Manually tested with Firefox 46.0.1, IE 11.0.9600, and Chrome 51.0.2704.  Previously rendering was fine in Chrome, but strange in Firefox and IE.  Now buttons look fine in all three browsers.