Revised for Default Theme (white background)

I've created my own version for **default theme**
It's basically revised from beginning...
So, you may want to check any new problem occurs..

Hopefully it can solve most highlight problems.
e.g. URL with underscore "\_", added code syntax  \`code\`, etc.

(current issue: still treated as Heading when type \#hashtag )

This is my first time to pull request to Github.
Please tell me if there is any problem (or bad practice :-)

Thanks.