You can edit the default backend theme's windows.css file and then map the edited theme from Administration >> Appearance >> BackendTheme. You can find the default backend theme in your SDK folders SDK >> Content >> Resources >> DefaultBackendTheme. In the windows.css file you will find all windows styles and there you can edit them the way you want.
Let me know if you have any problems with that.
All the best,
the Telerik team
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