Where to put <style> inside SVG?

It doesn't matter. Neither approach is "more standard". <style> elements are not renderable anyway, so there is no real need to put them in the <defs> section

As commented by Paul LeBeau.

After reading this article about style on MDN, that shows an example of a style simply under the SVG root, I am more convinced it is correct to put <style> there rather than under <defs>.

Also, since <defs> tag is indeed for reusable graphical elements that should be rendered, and <style> is not a renderable element, there's no point keeping it there.