When you create a table in Excel Online, you’ll get the default table style with banded rows. That is the only table style currently available in Excel Online, so you won’t be able to choose another preset table style or make any changes to the default table formatting.
If you have the Excel desktop application, you can open your workbook there and apply different formatting or choose a preset table style.
Click Open in Excel and follow the steps in Format an Excel table.
When you save the workbook and reopen it in Excel Online, you'll see the style changes you made.
For news about the latest Excel Online updates, visit the Microsoft Excel blog.