Question: I managed to find a block that supported setting of the margin, but which one?
Answer: Site Title and Site Tagline support custom margin.
It’s defined in their
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Spacing for the Site Tagline block
Here we can see that the Site Tagline block supports Spacing settings for Padding and Margin.
The default values are not shown. As soon as you start changing them the block changes visually. This may not be of any use to a person using a screen reader.
For the Site Tagline block, changing the top margin didn’t seem to take any effect until it was set to 9px.
Note: You also have to take into account the padding produced when you choose a background colour or gradient.