The article says clearly they are talking about distros that apply system wide themes without checking that all GTK applications still look good and not taking about people tinkering. Distros do this because it looks good in screenshots and give the system a unique flare, but it gives all these applications a bad image when people try them and they look terrible. Is the expectation that programs like geary should loo…
Apps should be designed with the goal of being themeable. Applications need to supply hints and avoid e.g. icons with baked in BG.
and even if they were designed to be themeable, that doesn't address many of the other concerns here - designing an app to be themeable doesn't automatically update the documentation with whatever custom themes get applied, or ensure that the icons you've chosen in your theme make sense in the context of the app. "apps should be themeable" seems to be the point of this manifesto - the app developers could just block themeing entirely if they wanted to. they don't want to do that, they seem to just be asking that if a distro wants to apply a theme to an app, then that distro should be the ones doing the QA to make sure the app still functions as intended with the custom theme, and that documentation is still available for users who experience the themed version of the app.