General Customization Question

I am brand new to Omeka, so I'm not sure how completely customizable it is yet. My question is, can I customize a theme to look almost exactly like an html site I've already created? My client loves the look of his existing site, and his business cards match, but due to the volume of antiques he has on his site, I'd like to take advantage of a database that catered to his area of interest.

Have you looked at the Showcase? You can see that there are a variety of designs created using Omeka:

Omeka is structured in a way that you can control the design of the site through themes--either by modifying one that we provide or by starting one from scratch. The themes use a combination of HTML, CSS, and PHP. So, it really depends on what you are looking to do, but I'm guessing it is possible to create a theme that will look very similar to the current site your client uses now.