Make a primitive box
Set its color with the liner gradation
90 degree is the best. And, the end color and the start color is the same color. Then, adjust the color saturation of the end color for your hope. I always set 21%.
Set the line styleI can't explain this factor in English well, because I don't know English edition of OmuniGraffle. See the following screenshot.
Add a primitive box
Sets its color with the liner gradation
This gradation is for transparent expression.
The start color and the end color is the same white color. And, make the end color transparence:
Remove outline of the additional box
To remove outline and get a roundness box, make the line color transparence. You should not set zero to line width, for a roundness box.
Adjust each dialogs for your best.
Save your dialog as a group. And adjust its colors.