While legal considerations might be your biggest worry, making your site more accessible is simply good customer service. More than 39 million Americans are blind and another 246 million have "low vision," Another one million are deaf in the U.S. Add to that people with mobility issues that prevent them from using their hands and that's a huge portion of the country's buying power.
WordPress themes designed with accessibility in mind tend to be easier to use and clearer to navigate than their contemporaries. While you may have specific goals in mind like ADA compliance or WCAG 2.0 standards to pass, rest assured that you won’t have to sacrifice on design quality. You’ll likely find a theme with a higher standard of design by searching for templates with these requirements.
The guidelines are broken down by four main principles: perceivable, operable, understandable, and robust. Under each principle, there are guidelines that provide specific goals a website should work toward. Under each guideline, there are testable success criteria. Those criteria are graded A, AA, or AAA. This grade shows the level of conformity to accessibility, AAA being the highest. The law only requires A and AA guidelines to be met.