Enhance .NET Excel Integration

I have an existing .NET application that needs three focused improvements.

1. Robust Excel input
• Parse and display data from both .xls and .xlsx files without errors or format loss.
• Feel free to use OpenXML, EPPlus, NPOI or another proven library, but the final solution must be royalty-free for commercial use.

2. Unified look and feel
• Replace the current UI font with Arial throughout every form, dialog and report so the interface appears consistent on any machine.

3. Reliable “Export to Excel”
• Inside the application’s print section there is an Export to Excel option that stopped working once we moved to Office 365. I need it fixed so it functions on every Office 365 install—desktop, web and monthly enterprise channels included. If Interop is no longer viable, implement a replacement (e.g., Graph API-based, OpenXML write-out, or another lightweight approach) that produces a clean spreadsheet mirroring the on-screen print preview.

Acceptance criteria
• Compilable source code with clear comments.
• Brief readme outlining new dependencies and setup steps.
• Demonstration video or screenshots confirming:
– Successful import of a sample .xls and .xlsx file.
– Arial applied across the UI.
– Exported file opening correctly in both desktop and web versions of Office 365.

The codebase is in C# with Visual Studio; I will provide the repository and sample files once we start.