HELP: categories menu template

[minuteago]

I spent all my day finding a similar issue, but couldn't find one. I want to modify Categories menu using simple and clean theme. One of the good sample sites I found from this forum is www.sexshop365.co.uk

 

I'm not asking how I can make categories exactly like the one I'm referring to. But, I want to know which php files(and where) should modify..

In addition to that, how can I draw a line between each category item?

In this case, from the website, the line between Clearance sale and new this month

 

I think I have to modify admin page as well... not sure.

 

Any help, I'll appreicate ..

 

Thank you