Steps to reproduce:
1) Add RadDropDownButton control
2) Load Office2010Black theme
3) Define specific System.Drawing.Font:

            Font font = new Font("Arial", 12.00f, System.Drawing.FontStyle.Italic);

4) Create RadMenuItem:

            RadMenuItem myRadMenuItem = new RadMenuItem();
            myRadMenuItem.Text = "My New Item";
            myRadMenuItem.Font = font;
            radDropDownButton1.Items.Add(myRadMenuItem);  

Expected result: change the font of the RadMenuItem
Actual result: nothing happens

WORKAROUND:
1) Open Visual Style Builder and load the Office2010Black theme
2) In the Controls Structure pane navigate to RadMenu - DropDown>> RadMenuItem and select the RadMenuItemTextPrimitive item from the Elements Pane.
3) Remove FontSegoeUI8pt and ForeColorBlack repository items
4) Select the RadMenuItem item from the Elements Pane and add FontSegoeUI8pt and ForeColorBlack repository items
5) Save the theme and load it in your application