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.