Help adding own stylesheet

I hate to be the one to bring this up again, but has the way you add your own stylesheet changed since the last update of PowerCMS? I have look back at the posts and the latest one which seems to be the most informative about this subject but I do not have the code in contents_insert.php or content_update.php that you refer to and so cannot edit it to include my styles.

I have the drop down menus for styles and format appearing in the CMS but just cannot work out how to add my own styles to them or delete any existing ones that will just confuse my client such as typewriter.

I did a search for typewriter in my site in the hope of finding the relevant stylesheet but instead found cms/webassist/ckeditor/plugins/styles/styles/default.js holds all of the styles mentioned in the drop down styles menu - is that how you are meant to add/amend styles now?

Could you help please, this site is meant to be going live tomorrow and I have got everything working except this (and another issue I have posted earlier about characters not displaying correctly).

Thanks