You don't need to "know" all HTML and CSS attributes. You just need to know how to XenForo templating system works and where to find everything (templates and style properties). If you don't know anything in HTML or CSS it's not a problem. You can always look it up. Same goes for XenForo, we're here to help
It's not very hard to learn HTML/CSS but you can slice and dice your photoshop template with photoshop. That'll automatically convert it to an html layout. From there you will need to know a bit more. CSS and HTML is a very good start.
Depending on what you need done you may also consider uploading an existing template and heavily modifying it to suit your needs. Xen makes it very simple to modify the template. You can change the header image, background, category images, colors, and more. There's not many ways you can get super crafty with forums without it not look like a forum >.> lol
So with that being said... I'd just read up on xenforo styling in their help section and replace what needs to be replaced to achieve your look and feel.