How to connect different directory files?

Hi

I am making a static site (A lyrics site) with many pages. I make a index.php file and split it in header.php, index.php and footer.php files. I connect the files with php "include" function.

now i make a directory called a-z in the root folder. Inside this directory i have 26 php files like a.php, b.php, c.php, d.php .... z.php and another 26 directory called a, b, c, d .... z.

Now when i connected this a.php, b.php, c.php etc files in the index.php the css is not working. But i can see the header content, footer content. just the CSS is not working. i think there is some problem with connecting the CSS.

Can any body help me with this?

and what is the directory connecting code? i am using this

include ('../_header.php');

Code (markup):

for connecting.

here is a demo >> http://thelyricsbank.com
Also check the Folder Tree here >> http://thelyricsbank.com/folder-tree/

really appreciate your help.

Thanks