I have noticed a big difference in how the system responds when changing the theme or customizing a theme in the Appearance dialog. In Ubuntu MATE 14.04 or 14.10, when a selection or change is made, results are seen immediately in all visible windows and the panels. There is not even an “Apply” button to press! This is good.
In contrast, in 15.04 there are no visible changes unless I log out (which closes my open windows) and log back in. It is obviously a lot slower and cumbersome to need to do this to see the results of each individual change.
I am wondering if this is the intended behavior of 15.04? If so, it makes the other releases definitely preferable in this regard.