Reading carefully between the lines, I think they actually do hint at that:
> Please don’t theme our apps
> This is why we ask respectfully that our applications not be themed.
> On a platform level, we believe GTK should stop forcing a single stylesheet on all apps by default.
> If you are a distribution who changes the system stylesheet and icons, please reconsider this decision.
> If you do, we ask that you please stop theming our apps.
Reading carefully between the lines, I think they actually do hint at that:
> Please don’t theme our apps
> This is why we ask respectfully that our applications not be themed.
> On a platform level, we believe GTK should stop forcing a single stylesheet on all apps by default.
> If you are a distribution who changes the system stylesheet and icons, please reconsider this decision.
> If you do, we ask that you please stop theming our apps.