Megalomaniak, no, it's from Steam. I don't know of any Linux distribution (or other-kernel'd OS) that programmatically installs, configures, and downloads packages from Steam in their default image! Perhaps that's why it's not detecting it properly - Krita actually has the same issue when installed from Steam in restrospect: https://discuss.kde.org/t/why-doesnt-krita-on-steam-adhere-to-the-system-theme/5371/1
I'll report it.