KDE has a comprehensive suite of Qt applications which all adhere to the system theme. It also uses client-side decorations so all windows have the same decorations. Most popular Qt themes also have a corresponding GTK theme to make the rare GTK application (for me mainly Qalculate, Firefox, GIMP, and Gnome's Simple Scan) look and behave very similarly.

Fonts also all match in all desktop applications, and I have configured Firefox to ignore CSS fonts and use the same ones as the rest of the system.

I internally laugh every time somebody gets excited about a new dark theme for their application/OS - like they're coming out of a cave into the 19th century, while I'm living in the future.

There's a plasma-integration plugin for Firefox which makes scrollbars in FF match the Qt scrollbars! It also changes the Firefox scrollbars to click-to-warp instead of click-to-pgup/pgdn, to match with the rest of the system (I believe this happens in all GTK apps though).

There is only one inconsistency I have noticed - in Qt apps you can scrollwheel in inactive windows (configurable if desired), but sometime in the last year GTK changed so that this is no longer possible - I need to switch window focus to scroll GTK windows, except for Firefox. Also Electron apps stand out very much and feel unnatural to use - especially the theming and animations in the File/Edit/View menubar.