DrRacket uses the framework’s color schemes to colorize source text and other aspects of itself. See color-prefs:register-info-based-color-schemes for details on how to add new color schemes via "info.rkt" files.
Plugins can register modes via drracket:modes:add-mode. Each mode is visible in the Modes submenu of the Edit menu. Initially, DrRacket only supports two modes: Racket mode and text mode. (The Racket mode consults the language in the #lang line; see Definitions Text Surrogate for more details.)
DrRacket automatically selects a mode for each open file based on the file’s extension (and the language chosen as described above). If the file ends with .txt, DrRacket uses text mode. Otherwise, DrRacket uses Racket mode.