How does Gmail change the theme background?

Gmail is Google's free Internet Mail Service, so how does Gmail change the theme of the background, the following is a small series of the tutorial

Open Gmail's Web page. Log in to your Gmail account. Click on the gear icon in the top bar. Expand the menu list and select Theme background.

Click on "Theme Background", in which the user can choose a different background to change.

In addition to Gmail to provide users with the background, users in the "Custom theme Background" column can also set their own favorite background.

In the "Custom theme Background" column, select one, click into the background image can be seen to add a variety of ways, there are: the way the local upload or network links. Choose a way to add the picture.

After adding the picture, click "Select". Then you add the picture becomes the background of the Gmail mailbox.