How to set CSS styles which only apply when using a mobile device?

I would like to change the font size for a field in such a way that these changes will only apply when I am using my mobile device. I.e. these settings should not apply when using Anki on a desktop. Does anyone have any ideas of how to achieve this?

Please see Styling & HTML - Anki Manual

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Thank you so much! I read this before but totally forgot.