I've found the culprit, during testing I'd read somewhere that I had to set the themename after initializing with this line:
Telerik.WinControls.ThemeResolutionService.ApplicationThemeName = "thmWindows7"
With thmWindows7 being an instance of a theme I dragged on the form; this caused the freaky visual style. When I removed it, it works great. The Windows7 theme name is now only selected in the ThemeName Property of the form.
Thanks for a quick response !