First up, this is good to know...

This means that theme authors who prefer PHP can now register custom block style variations without writing JavaScript code.

As for themes, my latest theme called Empt was built with the beta of WP 5.3 installed. I wanted to develop and test it with 5.3, so far it went well.

But to step out of 5.3 talk for a moment and on to the Gutenberg block editor...

One major complaint with the GB team, is that theme devs need a resource/reference to "every" block as it relates to customizing and overriding the core block "editor" styles. Creating themes with front-end block styles to match that of the theme is one thing, but the most frustrating and problematic step is to get those styles in the block editor views.

Too many times, you think you have the nested CSS pegged down, only to see the core style is still ignoring it. Then there's the fact every block in the editor is saturated with nested containers that combine the block but also the editor structures. There is also the issue that the styling (margins, paddings, etc) for block structure in the editor does not match a theme's styling (but that is a whole other story).

Having a nice reference where a theme author can simply copy a CSS snippet for every core block, and then make subtle style changes as needed. This would be a huge bonus and it could even garner more GB support by theme devs. Right now, theme devs have to guess and through trial-and-error, try to get some success with implementing their theme CSS into the block editor. The Classic Editor is a breeze.

I'm still playing with WP 5.3 and working on the next theme being built on 5.3, so we will see how things go.