It looks like it is not possible to increase/decrease indent level on PowePoint Web App using the latest version of Google Chrome and Safari.
Buttons are greyed and tab doesn't work.
Do you have the same problem?
Please try the same operation with a recent version of Internet Explorer.
According to the Browser Support sheet, Internet Explorer is better validated to support everything SharePoint has to offer:
Supported Internet Explorer versions:
The product group makes every effort to validate that SharePoint functionality works correctly with released versions of Internet Explorer. Customers who want a more deeply validated browser interaction experience should strongly consider Internet Explorer.
Internet Explorer 11, Internet Explorer 10, Internet Explorer 9, Internet Explorer 8