It seams that the "BG Image uses border margins?" option in button theme properties changes the CDATA CSS of the theme, but this doesn't seem to do anything on an iPad (iOS 7.0.4 and iViewer Next Build 288). I would expect the image to contract 50 pixels in each direction inside the bounds of the button. See the xml below or in the Gist. I wouldn't say this a terribly useful option, but why have it in the software if it doesn't work anyway? My petition would be to remove the option completely?
<theme type="button" name=".buttonWithBackgroundImageUsesBorderMargins_off[state='0']"><![CDATA[padding: 50px 50px 50px 50px; background-image: url(Square Favicon.png); color: White; font-size: 12px; font-family: 'Verdana'; font-weight: normal; font-style: none; text-decoration: none; text-align: center; vertical-align: middle; display: table-cell; box-sizing: border-box; -webkit-box-sizing: border-box; -webkit-tap-highlight-color:rgba(0,0,0,0);]]></theme>
<theme type="button" name=".buttonWithBackgroundImageUsesBorderMargins_off[state='1']"><![CDATA[padding: 50px 50px 50px 50px; background-image: url(Square Favicon.png); color: White; font-size: 12px; font-family: 'Verdana'; font-weight: normal; font-style: none; text-decoration: none; text-align: center; vertical-align: middle; display: table-cell; box-sizing: border-box; -webkit-box-sizing: border-box; -webkit-tap-highlight-color:rgba(0,0,0,0);]]></theme>
<theme type="button" name=".buttonWithBackgroundImageUsesBorderMargins_on[state='0']"><![CDATA[padding: 50px 50px 50px 50px; -webkit-border-image: url(Square Favicon.png) 50 50 50 50; color: White; font-size: 12px; font-family: 'Verdana'; font-weight: normal; font-style: none; text-decoration: none; text-align: center; vertical-align: middle; display: table-cell; box-sizing: border-box; -webkit-box-sizing: border-box; -webkit-tap-highlight-color:rgba(0,0,0,0);]]></theme>
<theme type="button" name=".buttonWithBackgroundImageUsesBorderMargins_on[state='1']"><![CDATA[padding: 50px 50px 50px 50px; -webkit-border-image: url(Square Favicon.png) 50 50 50 50; color: White; font-size: 12px; font-family: 'Verdana'; font-weight: normal; font-style: none; text-decoration: none; text-align: center; vertical-align: middle; display: table-cell; box-sizing: border-box; -webkit-box-sizing: border-box; -webkit-tap-highlight-color:rgba(0,0,0,0);]]></theme>