Thanks for writing in! With regards to presets, you're pretty much right on the money with how things will work. You will be able to tweak a button to get it just right, and then save that entire collection of of settings as a named preset (let's call it "My Awesome Button"). Then, when you add another button to your page, if you want to use that exact same style again, you can simply apply the "My Awesome Button" preset and it will be just like your previous button, making these styles reusable and predictable across your entire website.
Regarding the "editing" of a preset and having those changes propagate to every button on your website, there is still a little bit of discussion around this internally as technically a "template" or "preset" is something you would expect to be immutable and unchanging, but we can see the power in wanting to be able to adjust it from one spot and have it filter down to every element that has these things applied. While I can't provide any definite feedback regarding this part of the equation, you can be certain that our next big item does indeed include the ability for users to define and save their own presets, which can then be recalled as needed or moved to over sites if desired.
Hopefully that helps to provide a bit more context, cheers!