Actually, it's dismissive of the app developers' egocentrism. In so far as theming breaks usability, they do have a point. Distributions shouldn't make potentially incompatible changes to the default theme without QA. But it's not all the fault of distributions and not respecting branding is not a UX problem.
Yes, I may be too harsh towards people voluntarily writing free software. Developers putting their branding and their "vision" before usability and user control is just a major pet peeve of mine. And diplomacy is not my strong point.
> Even the slightest change in color can introduce friction between UI elements. Or to be more extreme, if a theme turns your entire app into a white blurry mess, you would get mad at the application, and not the theme.
Actually I would blame the theme, but yeah, the average user would. Wrongly assigned blame is a broader issue with free software and the distribution system though.
> On the topic of icons: just like language, they evolve over time.
The XDG Icon Naming Specification does not actually specify the names of concrete icons, but of meanings. For example, there is no "looking-glass" icon, but there is a "system-search" icon. If you use the "system-search" icon to display a looking glass because that's the metaphor the icon theme you're testing with uses, your application may display the wrong icon with a different icon theme.
If an application needs an icon that is not standardized, it can include it similarly to its own application icon.
> It's about being able to recognize the brand across different operating systems
No, it's about the user recognizing the application. Which they can. The Firefox icon the open letter used as an example is very much the Firefox icon, just in a different style.
> it's also [...] not designed for the purpose of user experience (but in a sense, they still are)
Exactly. User experience >>> branding.
