I noticed all syntax coloring for comments is green.

How can I change this to gray across all language syntaxes?

edit your tmTheme

<dict> <key>name</key> <string>Comment</string> <key>scope</key> <string>comment</string> <key>settings</key> <dict> <key>fontStyle</key> <string></string> <key>foreground</key> <string>#878787</string> </dict> </dict>

Where is the tmtheme file?

The default themes are in Packages/Color Scheme - Default.