I believe my work proxy is preventing me from being able to add themes and packages to Atom. From the preferences menu, I get:
Fetching featured packages and themes fai
I edited the ~/.atom/.apmrc file to set my proxy as mentioned by @AlexMooney, but still got errors. The solution was to write
~/.atom/.apmrc
proxy = http://host:port strict-ssl = false
in that ~/.atom/.apmrc file.