The problem is that themes are structured with certain expectations for what is to be presented: recipient information, reference numbers, line items, custom fields, etc. These elements are available on transaction forms like sales invoices or purchase orders. Reports are individually structured to present data from the general ledger, not line item information. Logos, addresses, and so forth just take away valuable space. When you read annual reports of publicly traded companies, the financial pages are typically very plain, just filled with numbers.
If you want other stuff, create a PDF and add objects. No need to put things in Excel first.