Hello Piret,
I'm Shalom and I'd happily help you with your question. In this forum, we are Microsoft consumers just like yourself.
Regarding your first question, currently, Microsoft Sway offers options when it comes to choosing the font as well as the size. However, the options are limited to "Subtle" and "Moderate" emphasis. As of my knowledge up to 2021, there isn't a feature where you can choose the font size separately for the Heading and text cards. Any updates or new features would be announced by Microsoft.
As for your second question, the background is solely determined by your design/layout choice and cannot be customized. This means that if you choose one design, it applies to the whole document. However, you can change the background image of a title card.
I hope this helps! Let me know if there's anything else I can assist you with.
Best regards,
Shalom