Hi Manu,
I think we are using the word tab to refer to different things. Since you are talking about an area with a color gradient I assume you are refering to the narrow band at the top which carries the tab labels. I am using "tab" in a more general sense to include the tab body which is a container cell for components.

Normally I am used to seeing horizontal or vertical boxes as being a gray color. But in a themed Windows 7 if I add them to a tab they are no longer gray but white. The question is where is this white color coming from. It seems to be propagating from the parent container which is the tab. If you look in the aero theme style sheet you can see that the colors correspond to various shades of white and off white. I don't see anything that looks like a gradient.

I am not rendering the SVG directly to screen, but to disk first as a png. This is then applied to a normal button using set_pixmap. This button is added to a horizontal box which in turn is added to the tab. 

If I make it transparent the color of the button underneath will then be showing. But there is another reason I do not like to transparent png's. From past experience Vision2 does not do a good job of anti-aliasing the transparent pngs. But this was 3 or 4 years ago. Maybe things have improved recently?

Also I want to make my graphics to be a fixed size irrespective of the user's monitor dimension/resolution. To do this I need to resize the png for each user monitor and from past experience Vision2 resizing looks a bit ragged. But perhaps things have improved lately? I don't have these problems if I use SVG as the png source.