Regarding your question about RadGridView indeed you should modify the FilteringControl.xaml, however I am not quite sure what might be the reason for this behavior. Would it be possible to send us a small runnable project with this theme as an attachment in a new support ticket? We may take a look at it and we would be able to provide you with an appropriate solution.
In order to get your chart styled properly, you need to edit the chart.xaml file in your custom theme. Please follow the instructions explained in this help topic. The element which represents the horizontal scroll bar is actually a customized RadSlider and the style which needs to be edited is:
All the best,
the Telerik team
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