After you have created your event, you can update your theme in two places.
On the design tab:
Scroll to the bottom of the design tab, and edit your theme. It will automatically save your changes.
You are able to update the logo and the theme color (all the way down to your preferred Hex color code).
Note: if you save the theme as the default, all future events in this channel will use this theme.
Inside the conference room:
When your conference is in progress, scroll your cursor over the logo in the upper left hand corner.
Click EDIT THEME.
Then edit your theme. Make sure to click SAVE for the theme to save.