Customizing OWA themes is quite simple to do. Here's an example of a theme I just put together, I call it "MOOWA":

The fun part about this theme isn't so much the graphics as the sounds: included are a "mooooooooo" for the new mail sound as well as a cowbell[1] for the reminder sound.

For more information on customizing themes, here is a comprehensive article. But if you want quick steps on how to get the above one up and running, here they are:

  1. Log into OWA as a test user with IE. Go to the Options page and choose the "Silver" theme.
  2. Download the zip file and save it to your server. Unzip it.
  3. Copy reminder.wav and notify.wav to C:\Program Files\Exchsrvr\Exchweb\Img, or wherever Exchange is installed on your server. Make sure to make a copy of the existing ones first.
  4. Go to C:\Program Files\ExchSrvr\Exchweb\Themes
  5. Make a copy of the "2" folder and name it 2.original
  6. Copy the files from the zip into the "2" folder, overwriting the ones already there.
  7. Use notepad and open "OWAColors.css" in the "2" folder and make the following changes:

Explanation Before After
Puts the cowprint behind the list of messages .msgViewerCont, .msgViewerGroupCont
{
 background-color:white;
}		 .msgViewerCont, .msgViewerGroupCont
{
 background-color:white;
 background-image:url(/exchweb/themes/2/logo.gif);
}
Makes the bar above the message view black. .fldrTitleBar
{
 background-color:#706F90;
}		 .fldrTitleBar
{
 background-color:#000000;
}
Makes the background behind the MOOWA logo in the upper-left black. .nbTableMain
{
 background-color:#706F90;
}		 .nbTableMain
{
 background-color:#000000;
}
Makes sure that the column you're currently sorting on still shows the background image .vwSortCol
{
 background-color:#F5F5F6;
}		 .vwSortCol
{
}

All that's left to do is Ctrl+Refresh IE while logged in as the test user from step 1 to make sure everything's kosher.

[1] While testing the theme, I demo'd the sounds for my husband. He said "Play that one again" about the reminder sound, which I did. Then he said "It needs more cowbell."