Display Properties > Programs. A new page chooses what GTK and Qt programs wear: the GTK theme, the icon theme (with a few of its icons shown beside the box) and the Qt style, each listed from what is installed. Chicago95, Chicago95 and Windows are the defaults, so nothing changes until you change it. The choice is kept in the scheme and written, together with the colours from the Appearance page, to the GTK 2, 3 and 4 settings and to qt5ct/qt6ct on Apply and at every logon. Programs pick it up when they start.
curl -sL jacktulli.github.io/w2k | sh