You could add a theme file at the top of the view file with other JS files loaded there. Demo:

<script src="http://code.highcharts.com/highcharts.js"></script>
<script src="https://code.highcharts.com/modules/exporting.js"></script> // modules

<script src="http://code.highcharts.com/themes/gray.js"></script> // theme

@using Highsoft.Web.Mvc.Charts // and .NET wrapper code goes below...

General info about including JabaScript files is provided in the .NET wrapper documentation.