To achieve different than the default behavior of a control in its visual states, you should override its template. In your case you should edit the RadRichTextBox's Disabled state.
For your convenience, I've created a sample project which illustrates the desired visual effect with the above described approach. You can find it attached. In addition, I've also left some "NOTE
" comments in the App.xaml file, so you can easily see what is changed in the template and compare it to the default RadRichTextBoxStyle.
Please note, implicit styles are used in the demo project as this is the theme setting approach which we recommend. More about this topic, you can read in our help article here
If you have further questions, I'll be glad to assist you.
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