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.