Hack: v2.3 Login style w/ v.2.5

I am a newbie to WP. I recently upgraded from 2.3 to 2.5. I did not like the new login style and there were bugs in the style sheet as noted elsewhere in this forum. I modified wp_login.php to use the new code but the old style sheet.

My questions are … is that cool? What is the downside? Is there a right and wrong way to modify the code? Does anyone care about this – Is this the sort of thing I would share with others, and if so – how?

I realize these are newbie questions. The docs on here are vast and I couldn’t find the answers. Sorry. Please be gentle with the new meat.