Livery themes a whole Windows desktop at once: widgets, layout and colours change together. Nine themes (three free), readings for CPU, GPU, memory, network and drives, and a drag-anywhere layout editor. Widgets pause during fullscreen games. No account, no telemetry. Free on the Microsoft Store, with an optional one-time Founder Pro (about $9.99).
I wanted a theme to mean a whole desktop, not a loose pile of widgets and wallpapers. I made NotchiDex first, and Livery is my second Windows app.