There are themes for Invoices and some other transactions, but is it possible to applythemes for reports?
No. Other than customer and supplier statements, reports are for internal use or to share with bankers, auditors, investors, and the like. By long tradition, accounting reports are very plain. If you look at annual reports of large, publicly traded companies, the glitzy parts are in the front usually. When you get to financial statements in the back, those are almost always just columns of numbers with necessary labels.