I was able to change my GTK theme with
lxappearance but would prefer to know the command (or multiple commands, if GTK2 and GTK3 themes need to be set separately).
At the moment, I'm on an Ubuntu derivative with i3wm but I was curious whether there is a general command/answer--other answers on the Internet appear outdated. I had tried setting
org.gnome.desktop.interface gtk-theme but I imagine that only works in GNOME environments (not i3wm).