For the background image, this depends on the theme you're using. If it's not a feature of the theme, then you could achieve it on your CSS.
You can customise the text of your log in screen at [yourmoodlesite]/admin/settings.php?section=manageauths in the 'Instructions' field.
But for mobile how wee can do that behan
do you want to add that page in your custom app or in the official app?
For the official app that's not yet possible but we are working in a way to do it (once enter your URL, the CSS file will be loaded)
I want to do it in offical app..I will wait until you finish it