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A COSMIC Thanksgiving
(blog.system76.com)
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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
Community icon by Alpár-Etele Méder, licensed under CC BY 3.0
You don't seem to realize that this is equivalent to that. The user already made the choice to install a desktop environment which generates themes. So if you make the choice to build an application with GTK, and you want users to be able to use system themes with it, then consider it done.
To argue otherwise would make you a hypocrite. It would mean that you don't actually want users to use themes, so you take issue with desktop environments which make it easy to do so by default. So if you want people to be able to use themes, then you shouldn't complain when people choose to use a desktop which enables that use case.