New Post: How to remove table theme?

theme property must be set as:
ws.Tables.First().Theme = ClosedXML.Excel.XLTableTheme.None;

So, your code must look like this:
ws.Tables.First().ShowAutoFilter = false;
ws.Tables.First().ShowRowStripes = false;
ws.Tables.First().Theme = ClosedXML.Excel.XLTableTheme.None;