If you want to edit a reminder message window, use Message Window Editor. You can open it from Reminder Settings window (click Edit Message link, see more details about it in Edit Reminder section).

Using Message Window Editor you can completely change message window view: colors, fonts, image, message size - all is customizable. The easiest way to change the view is to select a predefined Scheme from the Scheme List. It is also possible to create a new user defined view (you even can save it as a scheme using Save As.. button).

How to use Message Window Editor:
Select an Item you want to change (you can select an Item using Mouse or you can pick it from the Item List - see the image below).
After you select an Item, use item properties editor to change selected Item settings.

To see the resulting Message Window view, press Preview button.

To save changes - press OK button; to discard changes - press Cancel.

There are more settings that can be defined for all message windows:
Stay on top - if message windows should be at the front of all other windows.
Show taskbar button.
Transparency.
Position.
To define these settings, refer to the Options topic.