I use a light theme, but the background of my Editor windows is dark gray with a white text font. This also has effect on the Command window. But when I now execute a command and an error is raised, the error message is shown in black, so it is hardly readable on my dark gray background. Can the error message maybe use the same font color as selected on User Interface/Fonts/Editor?

We'll see if we can enhance this.

Can you send a screenshot?

See attached screenshot with the error message "ORA-00900: invalid SQL statement".

You can go to Preferences > Windows > Program Window and set the "Error text" color. By default it will be red.

OK, I have indeed set the error text to black, but the 'Error background' is set to red. Shouldn't this background color be used as well then when an error message is shown?

The command window only uses the error text color.

Aha, then I guess I will have to try to enhance this myself...