I am creating a presentation in Beamer, and I am using a theme that I really like except for the fact that the boxes it creates for the theorem, definition, etc environments are white. I was wondering if there was a way to change the color of these without choosing a new theme?

Of course it is possible. There are several levels on which you can adjust the colors. I'm not an expert, but maybe the header of my latest talk (in which I do play with colors) is a help for you; the places in which colors are modified are commented.

\documentclass[xcolor=dvipsnames]{beamer}

\useoutertheme{infolines} 
\usetheme[height=4mm]{Rochester} 

% Colors theme
\usecolortheme[RGB={20,60,70}]{structure} 

\setbeamertemplate{blocks}[rounded][shadow=false] 
\setbeamertemplate{items}[ball] 
\setbeamertemplate{navigation symbols}{} 

% A customized alertblock (hand-picked colors, used with \begin{altertblock} ... \end{alertblock} )
\setbeamercolor{block title alerted}{fg=white,bg=black}
\setbeamercolor{block body alerted}{fg=white,bg=black}

Thanks, I was able use that to figure it out.