Themes are backgrounds for Super Mario Maker and change the Ground blocks. There are also Themes for everything, it also will change Mario.
Themes consist of:
Land - The default theme. (Mario does not change here)
Ghost House - A spooky ghost-house theme. Ground blocks change into brownish-grayish mansion-like blocks. (Mario does not change here either)
Underground - A cave-themed theme with stalactites hanging off of the top. It looks like a space-themed therein Super Mario Bros. 3. (Mario still does not change)
Airship - A ship-like theme where ground is replaced by wood blocks. If enough are placed, windows/cannons may be seen. (Mario does not change once again)
Castle - A castle-like theme where lava is the floor and ground blocks are replaced by bricks. (Mario STILL DOES NOT change)
Underwater - An underwater-like theme where ground blocks are replaced by sea-bottom blocks. If they are stacked directly upwards and there is 2 or more stacked, the blocks transform into coral blocks. Mario also changes: Mario will swim instead of walking on ground. If Y is held while swimming, Mario will proceed to swim faster.