Is it possible to customize fonts / color of messages?

In the Mac client for Slack, I'd like to be able to change the fonts and colors for messages (e.g. have all messages / text in the message area be white text on a black background, or be a larger font, etc.).

Unfortunately, editing the index.html CSS under the Content/Resources directory in Slack.app only changes aspects of the splash screen.

Is it possible to modify these styles, and (if so), how would I accomplish that?

P.S: I wanted to create a slack tag, because apparently one doesn't already exist. But my rep is apparently too low. Sadness ensued.

As far as I know, Slack for OSX does not support customization of fonts or colors in the message area. You may make the font larger or smaller via the standard <Cmd>+ and <Cmd>- keystrokes.

You may, however, customize the sidebar portion of Slack, on a per-team basis.

From the Zendesk help page:

Choosing a new theme

1. Open the Slack team you’d like to customize.
2. Go to the Team Menu by clicking the down arrow next to your team’s name.
3. Select Preferences.
4. Click Sidebar Theme.
5. Choose a sidebar theme. To select custom colors, click customize your theme and share it with others.
6. When you’re finished, click Done.  

This will not change your message window, just the sidebar theme.

Slack font can be changed as below

/slackfont font name

For example, to change to Arial font:

/slackfont Arial

To reset to default font:

/slackfont

This changes the font style(not color) for entire slack including message area and sidebar.