Let us check how to create theme in screen Personas 3.0.
Click On P icon
Now -> Click on the themes as show in snap shot
Enter the Theme name and description
Select the general option from drop down list as highlighted.
Now to set the background image for theme -> Click on the icon highlighted.
Now select the image form the Image gallery and click on Done.
Now if you see then the background image will appear in your transaction.
Let us now set some size of user area in our screen.
Click on save and now your theme is ready to use.