To reproduce:
1.Add a RadRibbonBar (with several groups and buttons inside a group) and a theme (i.e. TelerikMetro).
2.Apply the theme for the RadRibbonBar: setting the ThemeName property to "YourThemeName".
for example: this.radRibbonBar1.ThemeName = "TelerikMetro";
3. Run the application and resize the form in a way to collapse all ribbon groups. When you open the popup for the group, all popup elements are with incorrect style.

Workaround: load the theme in the form's constructor.