Gmail introduced Custom Themes, Now Set your Own Background Images
Google introduced new custom themes feature for gmail users with two options dark and light. When Gmail unveiled new look back in November introduced a number of beautiful HD themes. But now even more personal touch by setting your own background.
You can upload your own images directly, select from your Google+ photos or simply paste any image URL. Or, if you don't have a particular image in mind, you can browse searchable Featured Photos section to find one that speaks to you. With so many great options, it may be hard to choose, so to get a sense of the images available, try searching for ‘hdr scenery’ or ‘bokeh wallpaper' in the Featured tab.
Google implementing custom theme feature gradually to Gmail users. If you don't get custom theme feature till now please wait few days.