luci-theme-openwrt: update -webkit-appearance

Compliment '-webkit-appearance'
with 'appearance', as it is
non-standard and has since
been succeeded by 'appearance',
accepted by all baseline browsers.

Future consideration to then
remove '-webkit-appearance'.

Signed-off-by: Joshua Criss <owlsy@outlook.com.au>

themes/luci-theme-openwrt/htdocs/luci-static/openwrt.org/cascade.css

@@ -663,7 +663,8 @@ img.cbi-image-button {
 	color: #000;
 	display: inline-block;
 	font-size: inherit;
-	-webkit-appearance: none;
+	-webkit-appearance: none; /* nonstandard, should remove in future */
+	appearance: none;
 	background: #fff;
 	text-align: center;
 	font-weight: bold;