To customize the background image of your cabinet select the menu icon (three horizontal bars) in the upper left corner of your cabinet. From the left menu select "Customize" and then select "Cabinet Background". From here you have the option of selecting from an included background, taking a photo, or choosing an image from your photo library.
From the left menu you can also change the icons displayed on your cabinet home screen by selecting "Customize" then "Edit Cabinet Icons". On the "Edit Cabinet Icons" screen you can move icons up/down by pressing and holding. You can also swipe to remove a particular icon. Don't worry about removing one as you can add it back in using the "+" button.
To remove any unwanted categories go to the cabinet left menu by selecting the menu icon (three horizontal bars) icon or swipe right while on your cabinet home screen. Next select "Views" from the left menu. From the Views screen, select the "Categories" view. From the Categories screen you can swipe right to delete a category. If you decide that you would like to re-add a standard category (Inbox, Todos, etc) just select the "+" icon to re-add the category.