Click on "Settings" in the menu at the top of Twitter A sub-menu will appear below your profile picture. Choose "Design". You can select a theme or scroll down below the themes and choose to change your Twitter background image or change Twitter's design colors.
I hope you got your answer as you are willing to change your theme or background. But if you want to change your profile background then you have to click on "edit your profile" . It will appear a pen sign to edit your profile background . I hope you got what i wanna say Thanks