I am not a big themer so the easiest way for me to quickly change the whole look of the OS is to use the theme engine almost every Custom ROM uses. You simply install the app from the Play Store and select it in the Theme section in the settings, it’s that easy. Of course a Custom Rom is needed in order to do that.  Here are a few themes I found in the Play Store which I thought were very nice.

Cosmopolitan

Play store link Cosmopolitan

Crystal 2

Play store link Crystal

Flow

Play store link Flow

JellyBlack (also available in cyan/blue)

Play store link JellyBlack and JellyBlue_X

Popstyle (also available in red)

Play store link Popstyle blue and Popstyle red

Signature Blue

Play store link Signature Blue

ThyrusHolo Black

Play store link ThyrusHolo Black