When the 'External Editor' option is turned on the background color in the Arduino IDE is set to R22 G130 B153. While this is a nice color it is very difficult for me to read many of the keywords such as comments and type names.
It would be nice if either the temptation to read this code was removed or if the colors were selected to make it less painful.
For the former I'd suggest replacing the code editor with a list box containing the names of the project files. Double clicking would open them in the editor associated with the file extension. If the IDE monitors the files for changes it could also indicate in the list box when it detects a change.
For the latter, it would be nice if the background color were user selectable (possibly as an sub-option to the 'external editor' input, which was enabled when appropriate), and possibly it could default to a gray which would not clash with the foreground colors.