Linux is a kernel, and has nothing to do with themes. While some desktop environments and underlying technologies of Linux-based desktop OSes may expose some sort of theming capability, it’s equally valid if a Linux-based desktop offers no theming at all.
FreeDesktop.org specifications try to ensure different desktops’ basic features are inter-compatible, but does not address visual style.
KDE Plasma officially supports themes, color schemes, accent colors, and FreeDesktop-compatible icon themes, as applied to both Plasma itself and apps using the Qt and GTK toolkits.
Several app developers are calling for Linux distributions and downstreams to stop theming their apps.
elementary OS does not support user themes, and consequently supports unique branding and in-app theming capabilities.
GNOME offers no official support for themes.
LibAdwaita, the implementation of the GNOME Human Interface Guidelines and design patterns, does not support user or vendor theming. As such, it enables powerful and easy per-app custom styling.
There are efforts to improve vendor styling with downstream distributors, but that may look more like setting an accent color than a whole separate theme.