Menu buttons are made up of several parts:

Thumbnail: The image you see. By default, a still from the video that the button links to.
Thumbnail mask: Areas of the thumbnail that should be hidden.
Frame: The graphic shown around the thumbnail -- basically a border.
Frame mask: Areas of the Frame to hide -- usually the center area to let the thumbnail show through.

To use a different button (set of all these images) that comes with DVD Architect, simply select the menu button and then double-click one of the buttons in the Button window. DVD Architect will apply the thumbnail mask frame, and frame mask.

If you want to use your own image(s), select the menu button and then go to the Button Properties window. You can change the elements you want on the Media tab. Select the element you want to change, hit the down arrow to its right, choose Replace, and then navigate to your image. If there's an element you want to leave out (for example, you have an image you want to use, but don't need a frame or any masks), choose Remove from that element's dropdown.

[i]Is there a way to get more themes and buttons other than what comes with it or do I have to upgrade my software [/ui]

Sony added a bunch of themes (which include buttons) with the release of DVD Architect 4.5, but you can download some http://www.sonycreativesoftware.com/products/dvda/default.aspright here at their website[/url]. Honestly, if you're not keen on the ones that came with DVDA3, the newer ones probably wouldn't impress you much, either. ;-)

Personally, I find most canned themes / templates too generic for most projects. I make most of my menus from scratch customized for the project. You can make your own themes, but I'm a fan of custom highlight masks. (That's a link to a tutorial I recently wrote.) You essentially create two files -- one that represents your entire menu and one that represents the highlights (the actual buttons).

Another option is to create a Photoshop PSD file that includes all the elements for a menu (background, thumbnails, frames, masks). There is info in the manual / online help about constructing them. If you look around the net, you'll find that there are PSD-based DVD menus available for free and for sale. But you should note that most of these were developed for Adobe Encore. The format for these is similar, but not exactly the same, so if you get any of them, you'll need to edit them first so that they conform to DVD Architect's methodology. (Specifically, the layer naming conventions is different, and Encore supports some things that DVDA does not.)

Hope this helped.

Rob