Text margin on individual pages

I’m trying to change the text margin to 30px and also the width to 700px on my page http://roxi-lola.co.uk/about, I had the coding earlier but it’s disappeared and now I can’t find the thread about it!

I know it was to put <div class=”myuniqueclass”> in the text editor but I don’t know what the coding was to put in the css box!