@JamieO -

What is the plugin that you have wrote which uses that look-for-a-plugin-centric-css-file-in-active-theme approach? The “standardization” that is starting to come together for child themes only came about by people talking about it and - where possible - taking their own steps towards implementing it as you have done. As the community shows interest in various areas, the core development team has done a great job of extending / supporting these visions for the platform.

I've not written any such plugin. Plugin development isn't really my forte (yet, anyway).

Rather, if I need to style the display/output of a given plugin, I either read the documentation for the plugin, look for a CSS file in the plugin directory, or just view the rendered page source, to find out what CSS declarations (ID/class) the plugin is making. Then, I add those declarations to my plugins.css file, and style as I see fit.

I like this method, because it provides maximum control and a future-proof (i.e. upgrade-proof) means of controlling plugin style.

I'm not sure if or how standardization would come about, but I'd sure support it!