what type of file should it be? Should there be anything in the file?

just an empty file, or even an empty directory called 'disabled' will do

Oh! thankx mates, I am able to log in to my site now and I am looking for the corrupted plugin. :)

There's lots of ways to change it, I'm saying the information of which file the original css is written in is inconsequential to your quest for changing it.

Write your own rules for it.  eg. if the button class is 'elgg-button' and it's inside a container of 'elgg-login-form'

then write your own rules:

.elgg-login-form .elgg-button {

  color: black;

  background-color: white;

  border: 6px solid red;

  // etc etc

}

Thanks Matt for your quick reply.

The problem is I don't know where to put the rules...which file? When I search button.php, I couldn't find "elgg-button" nor "elgg-login-form". And if I write my own rules like the example you've given, which file to put this rule and will the elgg know the variable (e.g. .elgg-login-form, elgg-button) about the login button automatically?

I am very new to elgg na.

Thanks krub.

Take a look at this

http://community.elgg.org/plugins/839775/1.8.3/customize-css