If you know Html, Css, and a little Php, it is relatively easy to modify free css templates to AMS skins.
Unfortunately there is no tutorial on how to do this.
The basic Basics:
1. In the Templates section of the Admin panel you will see a link to "manage skins". This takes you to the skins page where you will see a list of the skins installed and a link to "create new skin". Click this link and name the new skin you want to make (base it on the default Skin).
2. Now go back to the "templates" page and select the new skin (its the drop down menu above the "manage skins" link). This will allow you to edit the new skin while leaving the default alone. Go to "settings" and also choose the new skin so you can see the changes you are making.
3. Follow the "edit css" link and Copy and Paste the css file into it.
4. upload the images to the image folder found in your new skins folder (found in the skins folder) via ftp
5. Ams skins are made up of three template parts: Header , Footer and Main....you need to divide your free template into those three parts, which should be easy as most css templates have those tagged in the code already.
6. For the header and the footer, remember to place all of the AMS codes from the default into the new templates.
7. Add the main part of the new template into the "Main" template, but place it between $this->header and $this->footer as this code calls the header and footer templates.
8. All of the other Bolded templates that you see on the "Templates" page are called into the "main" template by the use of this code: $this->output
All of the other templates you see on the "templates" page are blocks and are called by using this code: $Blocks->nameofblock.
9. For most of these free templates, there is a section tagged "content", this is the section you will need to replace with $this->output in the "main" template and you need to help modify the bolded templates on the "templates" page. You will use the Blocks for all of the code you need to go in the sidebars.
Go through the AMS Default template code and try to familiarize yourself with how things work before you attempt to re-skin your site
This is obviously a very basic description of what you need to do in order to re-skin your site with a free css template. If you want, you can PM me with specific questions and/or I can re-skin your site for you for a small fee.
Any ways, good luck with your new AMS site.
I have been using ArticleMS for the last 5 years, Have many sites using the script and have extensive knowledge on re-skinning the default installation of ArticleMS.