if (client.available()) { // client data available to read
char c = client.read(); // read 1 byte (character) from client
// last line of client request is blank and ends with \n
// respond to client only after last line received
if (c == '\n' && currentLineIsBlank) {
So, the client requested the main page. You didn't care what the client wanted; you sent it the main page.
The page references a style sheet, so the client requests that. You don't care what the client asked for; you sent it the main page.
And, you wonder why the style sheet isn't applied. I think I can see maybe why it isn't.