This happens both in Rio and Tokyo:
It seems that the wwDataGrid doesnt respect the Header styling. If I create a "headeritemstyle" which is used by grids and change the text color, it doesnt keep the text color - it is always black in the wwDataGrid. In a TStringgrid, using the same style, the text color changes.
Also, messing with the OverrideStyleSettings.Title.FontColor has no effect (the other title settings do seem to behave properly)
Also, it seems that the cells in the grid are using the "editstyle" when editing which is meant for TEdit, not cells necessarily. I think the correct style element for that is the "EditorLineFill" and "EditorBackground" StyleResources of the gridstyle.
It seems to work for me. The grid won't use Headerstyle if you have TitleButtons set to true. In that case it uses buttonstyle. Are you sure you have not set TitleAttributes.TitleButtons to true?
I didnt see the titelbuttons option. It was TitleUsesButtonFont to false that fixed it. The Titlebuttons were false.
Anything on the Cells?
Oddly, when I set the TitleUsesButtonFont to false and set the OverrideStyleSettings.Title.FontColor to something, it doesnt use that setting or the headerstyle - I think it uses the button font color (I set the FontColor to White and it was gray which is button font color. The headeritem font color was also white)...so there may be a problem there...
The cells use the virtual method function GetDefaultTextCellStyle: string; virtual;
to determine the style of the cells. I believe this defaults to 'editstyle'. You could subclass the grid and override this if you wish to change this default. There is a reason to keep the grid's default as edit style so that the text does not shift or change size when editing.