I did a little testing to confirm what I told you and found you need to use TextBoxCommandInputs for the headers because they support HTML formatting for the string. Using this can you define the text and formatting. For example, here's the code I used to create one of the text boxes that I added to the table for the header. It's the third argument that is setting it as center justified, a larger font, and bold. The last argument is making it read-only.
col1HeaderInput = inputs.addTextBoxCommandInput('col1Header',
'Col 1 Header',
'<div align="center", style="font-size:12px"><b>Column 1</b></div>',
1, True)
Here's the resulting dialog.

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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com