That doesn't look like the editor text area, it just looks like the postbit width? vB5 uses a responsive theme. So, if you change those widths, you may cause issues with users using other devices to view your site.

So, this area, which you are looking at "here" is called postbit area?

Also, how can i change my template to the same template that this forum uses i.e. vbulletin.org

Is this template called Blue Lagoon classic?

You can't, this site is on vB3.8

Is this reply the answer to question; whether i can install blue lagoon template on my site: http://vivapack.net/

Since its not publicly available: no, you can't. Even if it was publicly available, it would be for vb 3.6 and not vB5, so: no, you couldn't.

No mod or style for vB3 or vB4 will work in vB5.

Thank you