For the past month or so, I’ve noticed that the theme selector is not doing too much. Most of the existing themes are not changing anything, some are changing some parts, but overall everything remains the same.
Does the Themes do something else that I’m expecting? Does the behavior changed?
The theme selector does what it does. It changes the theme. But it doesn’t change the color palette.
You can change the color palette in your preferences https://meta.discourse.org/my/preferences/interface.
I think when “theme default” is chosen the palette switches when you select a different theme. But otherwise it doesn’t
Quite many of those are really similar. So I think it works as expected, but because of clones you don’t see too much differences.
Quite often differences comes from colours. But because colour handling is totally not user friendly, you don’t see any changes.
The explanation kind of make sense, the functionality doesn’t 
Adjusted the theme on admin side, things are looking good now. Thanks!
I think it depends on what you use themes for. Themes like the amber one are the same as the default theme when the user selects a color palette. So when you use themes to offer different colors, it makes sense to force the users to use the default palette by not allowing them to choose color palettes because switching those can be confusing.
But when you offer different functionality, like horizon or redditish, which show the first part of the post on topic lists, while default does not, it can make sense to allow the user to select both, the features and the color.
There is also a color selector theme component Sidebar Color Palette Toggle