Hi Steve,
It's no problem.
You will find this community very helpful and supportive and full of people who are only too willing to help and provide advice. One of the main reasons I spend time on here is to repay some of that support I had when I was starting out with Moodle Themes.
I would suggest not changing the theme itself, but using Mary's How to clone a theme tutorial (So that your changes do not get overwritten by any updates, especially if you are using one of the core moodle themes) - or creating a new very minimal theme that uses the original one as a parent and just contains the css changes you need (Using a parent means that any changes/updates/bug fixes to the parent theme will automatically be included in your theme). You should find all the information you need when you read the links I gave you, but don't hesitate to come back to the forum for any help and support about anything you don't understand in what you are reading or trying to do.
Have fun moodling
Richard