In your case by changing the Background color of the selected GridViewCell affects the whole GridViewRow because by default the SelectionUnit property of RadGridView is set to "FullRow". This means that all the cells in that row are selected and that is why they are visually changed. In addition there are two additional unit types and different SelectionModes, which will also be affected by modifying the Background color of the selected cell. For more information about them, you can check the Basic Selection article.
A possible way to customize the style of the selected GridViewCell or GridViewRow is to edit respectfully the GridViewCellTemplate or GridViewRowTemplate templates. There you can customize the Border element with x:Name="Background_Selected" and the contained inner Border element.
Please keep in mind that if you need the SelectionUnit property of RadGridView to stay set to "FullRow" and change the style only of the current cell (which is focused), you will need to find the Border element with x:Name="Background_Current", which is located in the GridViewCellTemplate. Another possible way to change the Background color of the current cell is to use your approach, the only difference will be that you will be using the IsCurrent trigger property instead of IsSelected.
For more information about themes and how to edit a ControlTemplate, you can check the Setting a Theme (Using Implicit Styles) and Editing Control Templates documentation articles. In addition you can check the Override the Default RadGridView Styles article.
I hope this helps.
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