If you mean as in graphical changes, your answers are above.
But, if you mean modifying them, you can't do this normally.
However, you can use PGE to make a block bigger or smaller than it's normal size.
You can use PGE editor, make a block a different size and save it in PGE and you're done after reloading the level.
Don't save it in the Legacy editor (old SMBX editor), nor select/use it because SMBX normally doesn't allow custom sizes.
Now for backgrounds, if you mean the backdrop then you could use Lunalua and check out ParalaX, this allows you to modify backdrops with Lunalua.
But if you mean Bgos, I don't have a good clue of doing so.