Remember, it takes under 2 minutes to change a WordPress Theme. The design structure of your website is kept separate to your website's content. So by a simple click of a button you can change between themes without losing or grossly affecting your information. You don't have to spend weeks re-jigging the whole thing like you would have had to even just a few years ago.
Write down some initial thoughts, as to what you might require in a Theme, concerning colours, but more so layout, design and the number of columns. Many of them can be customized and adapted regardless of their initial colours.
Get a feel for how they place the Navigation bar, the width of the writing and the tabs relation to the rest of the site. How are the buttons arranged and display? Have a look at other pages than the Home page, what's changed, what's different?
In the SelfAssemblySites Members' Area, we have more information and videos on the pro's and con's of free and premium themes, when to use one or the other, some rules of thumb when trying to decide and some recommended theme designers.