FlashDevelop has support for UI and syntax themes and these can be combined to a full theme using a FDZ archive. These themes contains the UI theme files and syntax coloring override files to be merged to the language config files when the config files are read for the next time. These files are fairly simple and you should be able to make a full theme in no time.
UI theme files
The UI theme files are simple *.ini like files containing key and value pairs. The key usually is the controls type name and property name combined and the value usually is a hex color value. By default FlashDevelop offers properties for background and foreground colors but here are also some additional hardcoded keys to add more details. The active UI theme file is copied to "CURRENT" file next to the other UI theme files.
# This usually is the text color MainForm.ForeColor=#ECECEC # This usually is the background color MainForm.BackColor=#ECECEC
There also are methods in the MainForm for using the theme manager for custom needs.
void ThemeControls(Control); Color IMainForm.GetThemeColor(String); String IMainForm.GetThemeValue(String);
Syntax coloring files
The syntax coloring is part of the language config files but themes may override the existing color configuration by copying .override files next to the config files. These will be merged into one file when the syntax coloring is reloaded.
Examples and detailed info
See the Extensions page and the bundled themes for reference. See "Tools"->"User Config Files..." and then go into Settings\Themes.