Up to v3.2 the ix ag-grid theme applied the same background color to the grid, that is also used for the application, so it had the same appearance as the HTML grid.
See docs example here: Angular data grid - Code | Siemens Industrial Experience
This seems to have changed with the introduction of the new theming API in iX v4 and is also visible on the docs page: Angular data grid - Code | Siemens Industrial Experience
This visual change was not mentioned in the Changelog for v4 and therefor I wonder if it was intended? I personally preferred the old look, where the grid did not stick out so much.