gedit includes several different color schemes, allowing you to change the appearance of the main text window.
To change the color scheme, complete the following steps:
Select Edit ▸ Preferences ▸ Font & Colors.
Choose your desired color scheme.
The new color scheme will be applied immediately.
You can also use color schemes that have been created by others, or create and use your own color schemes.
Instructions on how to install custom color schemes, as well as examples of color schemes that you can download and use, are available on the gedit wiki.