This section is intended to give you a broad understanding of the possible ways by which you can change the visual appearance of the RadExpressionEditor and its elements. There is also comprehensive information about their ControlTemplates. You will need this information, if you want to customize their appearance.
To learn some basic knowledge on styling the controls, you can read the common topic on Implicit Styles.
There is also another way to obtain the template of a particular visual element. In the Controls installation folder on your PC, go to Themes folder (Themes.Implicit in case you're using NOXAML binaries) and select the theme that you use in your application. Then drill down to find the respective Telerik.Windows.Controls.Expressions.xaml file in that directory, like it is demonstrated in the Editing Control Templates topic. From this resource dictionary you can extract any needed resources.
Note that when changing the ControlTemplate, you should include all required parts. Even if your code compiles, some of the functionality may be subject to impact due to the omission of the required parts. The required parts are usually marked with the prefix "PART_". To learn more about the template of the RadExpressionEditor specific elements take a look at the Templates Structure article.