Added okaidia prism theme

Hi, I've created a prism theme! More information and examples here:

http://ocodia.com/post/okaidia-prism-js/

I've also dialled back the text-shadow as you suggested. Tried a letterpress effect, but it was so subtle it didn't seem worth it. Have yet to see it on a decent monitor, so feel free to tweak it if you see any improvements which could be made.

Love this highlighter Lea, great work!