To reproduce: 
-add RadDock with ToolWindow; 
-add RadGridView inside the ToolWindow; 
-apply Office2010Blue theme to the entire application; 
By default the GridDataCellElement has Font Segoe UI, 8.25pt with blue fore color. When the ToolWindow is floating, GridDataCellElement's font is changed to Microsoft Sans Serif, 8.25pt with black fore color.

Workaround: customize the theme by setting the appropriate font and fore color to the GridDataCellElement for the necessary element states.