Here is a simple trick which help you to remove yellow background behind your google adsense ads. You can also set your own color match to your theme if you like.

Remove the light yellow background from AdSense ads

Step 1.  Login to your wordpress via ftp or use file editor provide by wordpress Appearance ⇒ Edior.

Step 2. Now if you are using FTP then go to your active theme and open style.css to edit or in wordpress editor look for theme style.css.

Step 3. Find ins in your style.css and change background-color: #fff9c0; to background-color: #fff;

Save your style.css and check your page where you place your ads.