Hello realtec,
You can add such behavior to dropdowns by editing the theme file you are using and adding a transition rule to the .jqx-listitem-state-hover-themeName class definition like the following:
transition: background-color 0.1s ease;
However such trick will probably not work correctly for jqxGrid and possibly some of the other widgets, so try it on your own. You may need to inspect what classes are added on hover to the widgets you want such feature added, and this in itself could be a bit tricky.
Best Regards,
Vladimir
jQWidgets Team
http://www.jqwidgets.com