This applies to the desktop SDL version of SmallBASIC.
There are five color themes available in the text editor. You can switch between themes by pressing ALT+t while editing a file. When SmallBASIC starts the fifth theme is read from the settings.txt file (under %APPDATA%/SmallBASIC in windows and ~/.config/SmallBASIC in linux) and when it exits the current settings are written back to the file.
The steps below should give just enough functionality to help design a better looking theme. Sorry, it's a bit of a convoluted process.