Themes have been a big part of customization for the mod community. Themes became so popular that OEMs like Samsung started including a Theme Chooser. There are hundreds of themes in the Samsung theme store.
Marshmallow is slowly trickling out for Samsung devices. For now a few variants of the S6 and Note 5 have Marshmallow. If you are still waiting on your update you can get a taste of 6.0 with the new Marshmallow theme.
This theme is available for download in the Samsung Theme Store on the Note 5, Galaxy S6, S6 Edge, S6 Edge Plus, S6 Active, Galaxy A8, Galaxy S5 Neo, Galaxy J5, and Galaxy J7.