How about changing the style of the workbook?
var workbook = new XLWorkbook();
workbook.Style.Font.SetFontName("Open Sans");
that throws a NullReferenceException at ClosedXML.Excel.XLFont.SetFontName(String value) in c:\ClosedXml\ClosedXML\ClosedXML\ClosedXML\Excel\Style\XLFont.cs:line 320
at ClosedXML_Examples.HelloWorld.Create(String filePath) in c:\ClosedXml\ClosedXML\ClosedXML\ClosedXML_Examples\HelloWorld.cs:line 13
at ClosedXML_Examples.CreateFiles.CreateAllFiles() in c:\ClosedXml\ClosedXML\ClosedXML\ClosedXML_Examples\Creating\CreateFiles.cs:line 17
at ClosedXML_Examples.Program.Main(String[] args) in c:\ClosedXml\ClosedXML\ClosedXML\ClosedXML_Examples\Program.cs:line 8
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()