Change the color scheme
gedit includes several different color schemes, allowing you to change the appearance of the main text area.
To change the color scheme:
Press the menu button in the top-right corner of the window and select Preferences ▸ Font & Colors.
Choose your desired color scheme.
The new color scheme will be applied immediately.
For light and dark theme variants
There are actually two color scheme settings: one color scheme to choose when a light theme is applied to the whole application, and one color scheme to choose when a dark theme is applied. The Preferences dialog allows you to configure both.
About unsupported color schemes
Some color schemes may be unsupported for the theme variant used (light or dark). When that is the case, the color scheme cannot be selected and is marked explicitly as unsupported.
This happens when a color scheme inherits some colors from the theme, and it is assumed that the theme is either light or dark. If it were possible to apply such a color scheme when the wrong theme is used, it would appear that the color scheme has bugs and some of the text would be unreadable.
Use a custom color scheme
You can also use color schemes that have been created by others, or create and use your own color schemes. See the libgedit-gtksourceview documentation.