While Excel allows creating 3D charts, it doesn't offer direct formatting options for chart walls and floors like older versions. However, you can achieve some visual customization using the following methods:

1. Chart Area Formatting:

- Right-click on the entire chart area (not a specific data point).
- Select "Format Chart Area" from the context menu.
- In the "Format Chart Area" pane, you can adjust the fill (color, gradient, picture), border color, and shadow effects. These changes will apply to the entire chart background, indirectly affecting the perceived look of walls and floors in 3D charts.

2. Chart Rotation and Perspective:

- Click on the chart to activate the "Chart Tools" tab on the ribbon.
- Go to the "Layout" group and click "3D Rotation."
- In the "3D Rotation" pane, you can adjust the X Rotation, Y Rotation, and Perspective angles. Changing these values will alter the 3D viewpoint, influencing how the walls and floor appear.

3. Chart Wall and Floor Masking (Limited):

Note: This method has limitations and may not be suitable for all scenarios.

- Create a separate 2D chart with the same chart type (e.g., bar chart) but without any data points.
- Format the fill and border of this chart element to mimic the desired wall or floor appearance.
- Precisely position this element behind the 3D chart to cover the unwanted areas.
- This technique can be cumbersome and might not perfectly align with the 3D perspective.

Additional Tips:

- Consider using chart themes available in the "Chart Tools" tab to apply pre-designed color schemes that might indirectly affect the wall and floor appearance.
- Explore alternative chart types (e.g., 2D stacked bar chart) that might achieve a similar data visualization without the need for 3D elements.

Remember that formatting 3D chart walls and floors in Excel is not as straightforward as with other chart elements. The methods mentioned above offer workarounds to achieve some level of customization.