I had the same problem, the solution was to override css after loading the grid.
<sjg:grid
...
onCompleteTopics="loadCustomCss"
...
/>
Then in your jsp
<script>
$.subscribe('loadCustomCss', function(event,data){
$('head').append( $('<link rel="stylesheet" type="text/css" />').attr('href', '../css/grid.css') );
});
</script>
Where grid.css is your custom css, you can copy the css provided by the plugin, and this is especially useful when you downloaded it as a dependency on Maven.