If you are looking for a simple solution just to format the headers, you can do that in the Writer without the need of a template. I include an ExcelStyler before the writer to format the rest of the data, often keeping background white and text black. The ExcelStyler styling the cells other than the headers will overwrite any styling you do in the Writer for the headers.
But when it comes to templates it sounds like you may need to check the “Remove Template Sheet if Unchanged”. Also is the FeatureType for your writer named the same as the sheet/tab that you are using as the template? If not, try that.