This is not possible with Qlik Sense via the formatting interface that comes with he product. 1 option might be able to adjust the CSS of the table object as mentioned in the previous reply, another might be that you create a custom extension object that is styled as you see fit.
You can learn more about extensions here:
In the Developer Hub video - I create a simple table extension - it will give you an example of what is possible.
Let me know how you do.
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