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All predefined themes, including ControlDefault, assign specific Font to each of our control in order to guarantee consistent look of the whole theme. In this sense, there is not a single line of code that changes the Font of the entire application. To achieve the desired outcome you will need to set the Font of the controls on your form manually.
Typically, you will need to set the Font of the control, as is the case with smaller and simpler controls, e.g. RadButton, RadCheckBox:
.RadButton1.Font = customFont
.RadCheckBox1.Font = customFont
However, there are cases where you will need to set the Font property of the element as well. Currently, this is the case with RibbonBarElement. This is a considered issue and it will be addressed in a future release:
.Font = customFont
.RibbonBar.RibbonBarElement.Font = customFont
I hope you find this helpful. Let me know if you have further questions.
the Telerik team