> Sometimes I wonder why there isn't more enthusiasm around theming.
My guess: because it is difficult to develop software that can be themed and it is difficult to create themes that look good. Not only is it high effort, but it has relatively low returns. Themes mostly affect how things look and, ideally, have very little impact on functionality. I say ideally since, when there is an impact on functionality it is usually a negative one (e.g. buggy behaviour). Contrast that to a window manager or compositor: while it won't affect the functionality of individual applications (ideally), it does have a fairly significant impact upon how one interacts with the desktop as a whole.
My guess: because it is difficult to develop software that can be themed and it is difficult to create themes that look good. Not only is it high effort, but it has relatively low returns. Themes mostly affect how things look and, ideally, have very little impact on functionality. I say ideally since, when there is an impact on functionality it is usually a negative one (e.g. buggy behaviour). Contrast that to a window manager or compositor: while it won't affect the functionality of individual applications (ideally), it does have a fairly significant impact upon how one interacts with the desktop as a whole.