color - Changing background colour of a Manipulate element

Please consider the following:

 Manipulate[Plot[Sin[x (1 + a x)], {x, 0, 6}], {a, 0, 2}];

Now, I would like to change the background colour to yellow. I tried the following, but in this case the nice looking rounded corners of the manipulate element get lost:

Panel[#, Background -> Yellow] &@
 Manipulate[Plot[Sin[x (1 + a x)], {x, 0, 6}], {a, 0, 2}, 
  Paneled -> False]

Maybe somebody has a better idea.

EDIT

The background colour of Manipulateis by default grey.

In addition the graphical element in which the Manipulate is embedded has rounded corners which I would like to keep. Applying my approach will change the colour but I loose the rounded corners.

Answer

If you would like to change background of Manipulate function - this is a way. Note I'll show you most difficult case when you have to go from light to dark colors change - then you have to pay attention to styling of internal graphics too, so everything (like ticks) are visible.

st = {FontColor -> GrayLevel[.7]};
Framed[Manipulate[
  Plot[Sin[x (1 + a x)], {x, 0, 6}, Background -> GrayLevel[.3], 
   PlotStyle -> {Thick, Orange}, Frame -> True, BaseStyle -> st, 

   ImageMargins -> 7], {{a, 1, "frequency"}, 0, 2, 
   Appearance -> "Labeled"}, Paneled -> False], FrameMargins -> 30, 
 Background -> Black, BaseStyle -> st, RoundingRadius -> 10]