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.
1. Start SmallBASIC, then click to edit a .bas file, then press ALT+t four times. You are now looking at the modifiable theme.
2. Exit SmallBASIC (Alt+f4).
3. Copy settings.txt into a separate file (say settings.new), then open settings.new in a programming editor
$ cd ~/.config/SmallBASIC
$ cp settings.txt settings.new
4. Repeat: Make any changes to the hex values (see below) in settings.new, copy settings.new to settings.txt, and then restart SmallBASIC to inspect the changes.
$ cp settings.new settings.txt && sbasicg -e foo.bas
5. Once you have something looking fantastic, email me your settings and I'll include them in the next release.
--- settings.txt ---
Offsets for the above color hex values
0 - color
1 - selection_color
2 - number_color
3 - number_selection_color
4 - cursor_color
5 - syntax_comments
6 - background
7 - selection_background
8 - number_selection_background
9 - cursor_background
10 - match_background
11 - row_cursor
12 - syntax_text
13 - syntax_command
14 - syntax_statement
15 - syntax_digit
16 - row_marker