Unable to decrease the width.

I am unable to decrease the width of my forum..
When I go to ACP > General > configuration > in "page structure" there is an option of "forum width". Few days back I set the value 90% to this, but this time I wanted to set it to 80%. Hence, I changed it value from 90% to 80% but I found no change in its width. I cleared my cached and opened my forum again I found that there is decrement in the width, but ultimately the width again get increased as it was before (at 90%).
When I again visited to ACP >>> forum width option, there was no change in the value. Then I again decreased more width (and this time set to it 50% for testing) but same thing happened, firstly I saw the decreased width (to 50%) then ultimately it increased to 90% again.

Please anyone help me out.

Try to click save after changing it.

Ya that's I thing I don't forget to do because its written every "don't forget to click on save button"

width: 95px;
to your css under body

from where can I edit css ??

Ap >> Display >> Colors >> CSS
Find (for phpBB3)

Code:

 #wrap {
    background-color: #F7F7F7;
    border: 1px solid #666666;
    margin: 0 auto;
    min-width: 951px;
    padding: 5px;
    width: 90%;
}

Change the width.

Header width sometimes causes this kind of problems, wrap wont collapse cause header is too big. You can check that also.

But I have phpbb2 version

Sorry, you have phpBB3 under profile so I assumed.

This is for phpBB2.

Code:

.bodylinewidth {
    width: XY%;
}