Customization of Predeveloped Website

My website is made available to me by a third party company. They have given me access to modify certain parts of "my" website. I want to customize it to my needs. I've been able to do some of it but I'm having trouble with certain things such as getting the spacing of the menu items to look right, getting my "live chat" button in the right place, etc. It's all written in html/css. I need someone who can work on it N-O-W. Thanks...Jill.