You can customize the built-in Office2010Black theme using the Visual Style Builder tool. This help article demonstrates how to export the build-in themes and this one - how to save and load your custom themes.
To modify the Office2010Black theme according to your requirements, you could follow the steps:
1. Load the theme in Visual Style Builder and expand the control structure of RadDropDownList.
2. Select RadDropDownListElement from the control structure and DropDownListBorder from the Elements panel.
3. The ContainsFocus and ContainsMouse states of the element have assigned the DropDownListHoverBorder repository item. Increase the value of the Width property in this repository item - for example to 4.
4. Increase the Margin property of RadDropDownListEditableAreaElement to 4, 4, 0, 4 and the Margin property of RadDropDownListArrowButtonElement to 0, 4, 4, 4. It will increase the size of the control so the border does not hide its elements.
I have attached the updated Office2010Black theme according to the steps above for your reference. Please let me know if I can assist you further.
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