You can do that under Gtk3 too, but like with Qt5 it only allows small adjustments in line with how the developers built the UI. You can affect layout and style

You can do that under Gtk3 too, but like with Qt5 it only allows small adjustments in line with how the developers built the UI. You can affect layout and style of certain components, but that's it. From the perspective of making design decisions, it's somewhat equivalent to swapping the phone shell on a Nokia 3310.