Hello guo wei,
Thank you for the provided images.
To change the style of the selected pane you can extract and edit the default template of the RadPane. You can take a look at the Editing Control Templates
help article which describes how you can get the default template of the controls.
Looking at the provided images, I can see that you are using different theme. I am assuming that you are using NoXAML binaries. Can you share which theme are you referencing in your project? Also, do you have a custom style for the panes? If yes, can you remove them and check again if this behavior is still reproducible?
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