With the new changes, many variables were removed (like $primary-button-hovered-bg).
It is now generated through internal sass functions - but unfortunately, it only supports mixing color with black/white. resulting in washed-out colors if you want a lighter hover color.
I would expect to have an option to have a more linear lighten/darken sass functions.
Looking at the Kendo theme - try-lighten / try-darken exist that take the same input parameters.
There is also $kendo-solid-button-shade-function - but it is not marked as !default and cannot be overwritten.
Please add theme based on CSS variables. That would allow developers to change theme at run-time and offer our customers similar experience that most (large) web sites offers, like Gmail, Office 365 online ...
Also, in the long run, this could be the only theme that's needed. All others could be based on this one.