Hi, On 29 October 2018 (Monday), we are uploading our next LIB version (2018.3.1029) where the fix for RadComboBox will be included. It will also be included in our official version – R1 2019, scheduled for the mid of January 2019. In regards to System.Windows.Controls.TextBox, System.Windows.Controls.PasswordBox and RadWatermarkTextBox, you can set the ReadOnlyBorderColor property to desired color of FluentPalette. Here is the code snippet how to achieve it: SolidColorBrush brush = new SolidColorBrush(Color.FromArgb(255, 255, 255, 0)); Telerik.Windows.Controls.FluentPalette.Palette.ReadOnlyBorderColor = brush.Color; More information about FluentPalette can be found here: https://docs.telerik.com/devtools/wpf/styling-and-appearance/themes-suite/common-styling-appearance-fluent-theme Best regards, Ralitsa Kumanova
Note that this also happens with a regular textbox as well as with the RadComboBox.