Historically a lot of themes are designed for or are dependent on specific DEs. This is OK, but different DEs evolve differently. Mate and Cinnamon for example still have a lot of GTK2 dependencies. Other DEs like gnome are GTK3 dependent. Some application developers while not specifying a certain DE may spec GTK3 or GTK2 or QT 4 or 5 dependencies. Using one with the other may cause the GUI to act up with little things, or not work properly at all. Run a non-native application from the terminal some time without /dev/nulling the warnings and read them. DE theming itself, at least the way it is integrated/deployed now, is also changing because of wayland these days. In the future classic methods of deploying themes will change as Xorg becomes less and less important. Themes, like most things Linux, usually deprecate because of the work required to keep pace with development. I too liked the look of the arc theme in Linux Lite, but if it has ceased development there is no sense in offering it as a default theme option. Also i/e as developers have hardened OSs some themes have become unusable due to data leaks in Xorg. It's a heady but annoying time right now for graphics developers.
TC