Press the menu button in the top-right corner of the window and select
Choose your desired color scheme.
The new color scheme will be applied immediately.
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
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.
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.