You can change your theme by going to "My Account" then clicking the "Edit" link at the top, and then clicking "theme configuration" and selecting one of them. Then save your profile, using the button at the bottom of the page.
Note that for mobile devices, I recommend using the "noon" or the "midnight" themes. The image-heavy, spacious themes "art" and "houses" are only really suitable for large-format displays (recommend a 1080p, 24" monitor or better).
Noon (default, good for mobile and slow connections):
Midnight (good for mobile and slow connections):
Art (needs a large monitor with resolution up to 1920x1080):
Houses (needs a large monitor with resolution at or above 1920x1080):