RadSpellChecker uses pop-up windows to spellcheck the controls it supports. If you want to change some of the settings for these windows, like StartupLocation, Top, Left, TopOffset, LeftOffset, you can use the WindowSettings property of the static RadSpellChecker class.
The windows in RadSpellChecker derive from RadWindow, so if you want to check its properties, do it here.
Not all properties are exposed, so you can control only the ones that you have access to through RadSpellChecker.WindowSettings.
In addition to the properties inherited from RadWindow, there is a property called Theme of type Telerik.Windows.Controls.Theme. By setting it you apply a Telerik theme to the windows used by RadSpellChecker.
[C#] Example 1: Apply a Theme to RadSpellChecker's Windows
RadSpellChecker.WindowSettings.Theme = new SummerTheme();
You need to add references to Telerik.Windows.Controls and the Theme you want to use (in this case Telerik.Windows.Themes.Summer) in order to use it.