How to overwrite a thrid party block’s css in a child theme?

It would be of advantage if you post code to your problem. A concrete line of CSS with some context of the current Blocks in place?

First, you have to check which CSS is responsible for the look. You wrote child-theme TwentyTwentyFour but Third-Party-Plugins?

After that it is just a matter of specificity of the CSS-selektor and where you need to put it in place.

Usually, CSS can be added to the child-theme (head, inline, theme.json).

In case it is the CSS of the block/third-party-plugin, then this documentation is the one where to dig for a solution.