On the phone and tablet, the only appearance configuration should be setting the background pictures for the welcome screen, the home screen, or both.
In the “Background” panel, “Welcome screen” and “Home screen” should be thumbnails of the actual screens — complete with tint and infographic for the welcome screen, and Dash contents for the home screen. (This conveys whether, and if so how much, the current background makes each screen less readable.)
If “Different background for each” is set and you choose “Same background for both”, the most recently chosen background should immediately be copied to the other screen. Conversely, if “Same background for both” is set and you choose “Different background for each”, the least recently changed screen should return to its previously set background, if that background picture still exists on the device, even if it was last used as a background months ago. (This allows easy undo of syncing the backgrounds.) If that background picture no longer exists on the device, the screen should change to its default picture.
Whenever “Same background for both” is set, the welcome screen and home screen thumbnails combined should function as a single tappable button, navigating to a “Choose Background” screen. Whenever “Different backgrounds for each” is set, the thumbnails should function as two separate buttons, navigating to a “Welcome Screen” or “Home Screen” screen respectively.
All of those screens should show one or two sections: “Ubuntu Art” showing all pre-installed artwork (with the default image listed first), and “Custom” if you have added any images to the collection. The current background should have a frame around it. The toolbar should always be visible, with “Photo/Image” and “Remove” buttons.
Choosing “Photo/Image” should open an image picker. Choosing an image from the image picker, or an existing image in the “Custom” collection, should open a “Choose Area” full-screen dialog.
If the image was already present in “Custom”, by default the “Choose Area” dialog should show it at its previously chosen size and position. Otherwise, by default it should be centered and scaled to the smallest size that fills the screen. The “Best Fit” button should return to this scale and position, and be insensitive whenever the picture is already at that scale and position. The standard gestures should zoom and pan the picture to choose the area. If you do nothing for five seconds, gesture arrows should pulse into view in the center of the screen, hinting at how to zoom the picture.
Choosing a picture from “Ubuntu Art” should open a similar “Preview” full-screen dialog, but with no zooming and panning, and just “Cancel” and “Set” buttons, because that art is already optimized for the screen size.
The “Preview” and “Choose Area” dialogs should be unique in that the top bar, header, and toolbar should all be semi-transparent, so that you can see what part of the background picture will be visible when those elements are not present.
If you choose “Set”, the zoomed and panned area of the image should be added as the last item in the “Custom” area, with the screen scrolling to reveal it if necessary. It should be possible to add the same image more than once with different zoom/pan settings.
Choosing “Set” should set the background(s), and then navigate back to the top-level “Background” screen, because its thumbnails display confirmation of what you’ve done. No extra notification is necessary.
Following the standard deletion pattern, long-tapping on an image in the “Custom” set should reveal a popover menu containing a “Remove” item. If you remove the current background, the background should revert to the most recent choice that still exists on the device.