Add compatibility with Less 3.x

Any custom theme.config files will need to be updated to add the (multiple)
option when including theme.less.

src/theme.config.example

@@ -87,6 +87,6 @@
          Import Theme
 *******************************/
 
-@import "theme.less";
+@import (multiple) "theme.less";
 
-/* End Config */
+/* End Config */

src/theme.less

@@ -48,7 +48,7 @@
 -------------------*/
 
 .loadFonts() when (@importGoogleFonts) {
-  @import url('@{googleProtocol}fonts.googleapis.com/css?family=@{googleFontRequest}');
+  @import (css) url('@{googleProtocol}fonts.googleapis.com/css?family=@{googleFontRequest}');
 }
 
 /*------------------