css code:
/* keywords */
.geshi_code.php .kw1 {color: #b1b100;} /* language constructs (require, foreach, return) */
.geshi_code.php .kw2 {color: #000000; font-weight: bold;} /* constants */
.geshi_code.php .kw3 {color: #990000;} /* functions */
/* comments */
.geshi_code.php .co1 {color: #666666; font-style: italic;} /* single-line starting with "//" */
.geshi_code.php .co2 {color: #666666; font-style: italic;} /* single-line starting with "#" */
.geshi_code.php .co3 {color: #0000cc; font-style: italic;} /* heredoc and nowdoc */
.geshi_code.php .co4 {color: #009933; font-style: italic;} /* phpdoc */
.geshi_code.php .coMULTI {color: #666666; font-style: italic;} /* multi-line */
/* string escape sequences */
.geshi_code.php .es0 {color: #000099; font-weight: bold;}
.geshi_code.php .es1 {color: #000099; font-weight: bold;} /* simple single-char (\r\n\t) */
.geshi_code.php .es2 {color: #660099; font-weight: bold;} /* hexadecimal notation (\xAF) */
.geshi_code.php .es3 {color: #660099; font-weight: bold;} /* octal notation (\123) */
.geshi_code.php .es4 {color: #006699; font-weight: bold;} /* variables */
.geshi_code.php .es5 {color: #006699; font-weight: bold; font-style: italic;} /* cascaded variables */
.geshi_code.php .es6 {color: #009933; font-weight: bold;} /* format strings */
.geshi_code.php .es_h {color: #000099; font-weight: bold;}
/* brackets */
.geshi_code.php .br0 {color: #009900;}
/* symbols */
.geshi_code.php .sy0 {color: #339933;}
/* strings */
.geshi_code.php .st0 {color: #0000ff;}
.geshi_code.php .st_h {color: #0000ff;}
/* numbers */
.geshi_code.php .nu0 {color: #cc66cc;} /* integers */
.geshi_code.php .nu8 {color: #208080;} /* octal (0777) */
.geshi_code.php .nu12 {color: #208080;} /* hex (0xAF) */
.geshi_code.php .nu19 {color:#800080;} /* floating-point */
/* methods */
.geshi_code.php .me1 {color: #004000;}
.geshi_code.php .me2 {color: #004000;}
/* variables */
.geshi_code.php .re0 {color: #000088;}
That's just for PHP; each language has its own styles, and the class names used can vary.
BTW, when I said 17, I meant unique colors. You could have 27 different colors, but that's probably overkill. =p
I tried to comment the CSS with descriptions of what each class is for, but even I don't understand some of it.
/ 
