Have you encountered pale yellow background in Adsense or other advertising units plates in WordPress?

Then probable you must be using latest WP themes like Twenty-Fourteen. Well, the problem is not at all with ad-units., the actual culprit is the WP themes (Twenty-Ten, Eleven… Fourteen etc.). The color is defined in CSS tag ins of these themes. The solution is also pretty  simple. Search for the following lines

inside the theme style file

wp-content/themes/twentyfourteen/style.css

and replace with

That’s all, you are done! Now you have transparent background to all ad-units.

In continuation to this I would recommend not to edit the original theme files rather make a child theme and add the modified CSS ins definition there only. This shall prevent any overwriting in case of any future update of these themes.