question about css files

By nolox

Hello everyone, I was looking for internet but I didn't find solution for this two question:

1) What is css file controls login page. I wann change white background for an image background with a code like that:

body {
    background-size:100% ;
    background-position:left top;

I tried inserting it in index.php but the change affect to all website not only the webpage


2) What is css file controls the text "welcome username" appears in the activity page.


Thanks in advance