For the color theme, I suggest using a light-green coloring with yellowish white background. 

On the left hand side of We can put a logo. 

###Logo idea

We can use calligraphic writing of the Kalima Tawheed (La-ilaha-illallah..). The writing should be clear enough to be recognized the kaliman, meaning it should not be more art than it's message. I'm against rounding of the writing also. But, A half round circle writing may not be bad.

On top of the Kalima, We can use a palm tree and a crescent on the upper left side of the tree.

###Vote buttons
For upvote and downvote icons, I pretty much liked [yasar's answer][1].

[1]:https://islam.meta.stackexchange.com/a/61/177